There are disclosed a novel α-amino-N-(diaminophosphinyl)lactam derivative represented by general formula (1):
wherein, for example, A is a hydrogen atom or a substituent such as a lower alkyl group or the like; W, X, Y and Z are independently a hydrogen atom or a substituent such as a lower alkyl group, aryl group, arylacyl group, arylaminocarbonyl group or the like; and Q is —(CH
2
)
n
— wherein n is 0 to 3, or its salt, and a therapeutic agent and a prophylactic agent for myelosuppression, a therapeutic agent for infectious diseases, an agent for increasing the number of leukocytes and a dipeptidyl peptidase IV inhibitor which comprise the above-mentioned derivative or salt thereof as an active ingredient.