To provide an L-proline derivative having a DPP (dipeptyl peptidase) -IV inhibitory activity and a treating agent of diseases associated with the DPP-IV.

This DPP-IV inhibitor contains the L-proline derivative expressed by the following formula I [R1 is a halogen atom, an alkyl or the like; R2 is H, a halogen atom or the like; R3, R4 are each H, a halogen atom, hydroxy or the like; and X is NR5R6 (wherein, R5, R6 are each H, a cycloalkyl, a heterocyclic ring or the like) or the like] or its pharmaceutically acceptable salts as an active ingredient. For example, (S)-3-fluoro-1-[(2S,4S)-4-(4-phenyl-1-piperazinyl)-2-pyrrolidinylcarbonyl]pyrrolidine is cited.