A platinum (II) complex represented by the general formula: ##STR1## [wherein A's are each ammonia or two A's jointly represent 1,2-diaminocyclohexane, 1-amino-1-aminomethylcyclohexane or 1,4-diamino-2-methylbutane; and B's may be same or different and represent ##STR2## (wherein m is 0, 1 or 2; n is 0 or 1; R.sub.1 is a C.sub.3 -C.sub.6 alkyl group substituted with one or two hydroxyl groups) or two B's jointly represent ##STR3## (wherein R.sub.2 is a C.sub.2 -C.sub.5 alkyl group substituted with one hydroxyl group)]. These platinum (II) complexes have high potencies for use as an antitumor agent.