In this paper, I investigate grammaticalization from the viewpoint of dialect contact. The main claims will be that dialect contact leads to being degrammaticalized for an expression which has been defeated in a competition and being more grammaticalized for an expression which has won in the competition. Language contact occurs when two or more languages or varieties interact (See Winford (2003), Heine & Kuteva (2005), Frederic et.al.(2009) for more details). Frederic et.al.(2009) argues that when speakers of different languages interact closely, it is typical for their languages to influence each other. Languages usually develop by gradually accumulating dialectal differences until two dialects stop being mutually intelligible, somewhat analogous to the species barrier in biology. This paper shows that we can also find the dialect contact in Japanese. In fact, Kudo (2008) examines aspect markers of three different areas of Japanese dialects which are Western Japanese, southern part of Wakayama Prefecture and Okinawa. Jeong (2011) clarifies systems of tense/aspect/mood between dialect of Jeju island in Korean and one of Okinawa in Japanese. This paper proposes that differences of sika-nai ``only`` and hoka-nai ``only`` in modern Japanese can be also explained by dialect contact.