Preterm birth rate in Japan is around 5% of total birth in which majority of preterm birth has been between 32 and 36 weeks of gestation. On the other hand, perinatal mortality rate as well as neonatal mortality rate has been rapidly improved by progress of perinatal medicine. Neonatal mortality rate in extremely low body weight (ELBW) infant (<1000 g) has been improved from42.7% in 1950 to 8.1% in 2010 in Japan. However, approximately 50% of survived ELBW infants have neurological problems including cerebral palsy and/or mental retardation due to prematurity. Therefore, it is one of most urgent issues in perinatal medicine to reduce preterm birth especially between 22 and 27 weeks of gestation. Various examinations for high risk pregnancy and prognosis of threatened preterm birth have been introduced and measurement of cervical length (CL) between 18 and 24weeks is recommended by guideline issued by JSOG. Other than measurement of CL, several biochemical markers are also used for prognosis of threatened preterm birth such as fetal fibronectin, IGFBP-1, interleukin6, 8 and granulocyte elastase in majority of perinatal center in Japan. JSOG guideline also recommended examinations for bacterial vaginosis (BV) such as Nugent score and/or Amsel criteria before 20 weeks. Using these examinations, high risk group and prognosis of preterm birth could judge relatively correct. On the other hand, prevention and treatment of preterm birth are not established yet. Although many trials have been reported, it is difficult to compare clinical results since the sample group in each report is different. Among them, betamimetics such as ritodrine and magnesium sulfate have been used as tocolytic agents in Japan. It is generally accepted that ritodorine is useful for prevention of preterm birth within 48 hours, but the long-term intravenous administration is routinely performed in hospitalized patient in Japan. Magnesium sulfate is also frequently used in Japan with careful observation on side effects due to over dosage although clinical evidence for prevention of preterm birth is poor. Several RCT suggested that vaginal progesterone suppository might be more effective than intramuscular injection of 17alpha hydroxyprogesterone caproate. However, clinical trial of vaginal suppository of progesterone is just begun in Japan and there was no substantial evidence that treatment with progesterone is effective to prevent preterm birth in Japan. BV is considered as one of causes of preterm birth and JSOG guideline recommended to treat with metronidazole or clindamycine, however, clinical results by treatment with antibiotics are controversial. Similarly, JSOG guideline recommends to use “suitable” antibiotics when cervicitis and chorioamnionitis are suspected although there was no indisputable evidence. Administration of betamethasone is recommended by JSOG guideline to improve neurological prognosis of infant when preterm birth between 22 and 33 weeks is expected within one week as obtaining consensus internationally. Preventive cerclage after 12 weeks of gestation is one of alternatives for pregnant women with past history of preterm birth in JSOG guideline. Similarly, therapeutic cerclage is one of alternatives in pregnant women with short cervix at present pregnancy although a newest report indicates that therapeutic cerclage for short cervix does not prevent preterm birth. Examinations for diagnosis is almost established to find high risk group for preterm birth and prognosis of threatened preterm birth but further well-designed RCT is necessary to establish treatment for prevention of preterm birth.