This thesis deals with the problems on the pretextual arrest·detention and the investigation for other crimes. The pretextual arrest and detention indicates the kind of investigation used in some cases when there is no requirement for arrest and detention concerning the target crime. Some problems occurs on this kind of investigation in relation to the rule that every warrant must be issued on the particular crime. And there is another way of investigation similar but distinguished by the pretextual arrest and detention, that is the examination for other crimes. This kind of investigation is also not allowed in principle, because there is a rule that the period of arrest and detention should be used for the investigation of the listed crime. But there are some exceptions to this rule, applied when it is benefitable for the suspect and there is a reasonable cause for investigation for other crimes. And it is also a important problem that the period of arrest and detention for other crimes should be restricted or not, which is related to the fact that examining of suspect is one of the purpose of arrest and detention as a matter of fact. The period of arrest and detention for other crimes must be restricted by the reason that the investigation agency made already use of the period for the investigation of the same crime.