A platinum(II) complex represented by the general formula (A), ##STR1## in which R.sub.1 represents a hydrocarbon radical having 1 to 10 carbon atoms, R.sub.2 and R.sub.3 each represent a hydrogen atom or a hydrocarbon radical of 1 to 10 carbon atoms, or R.sub.2 and R.sub.3, together, may form --(CH.sub.2).sub.4 -- or --(CH.sub.2).sub.5 --, and X and Y each represent an ammonia molecule or a monodentate amine having 1 to 7 carbon atoms or X and Y, together, may form a bidentate diamine having 2 to 10 carbon atoms. The compounds are useful for treating a tumor.