The anti-cancer agent of the present invention contains at least one kind of phosphine transition metal complex selected from a group of compounds represented by the following Formulae (1a) to (1d). According to this anti-cancer agent, an anti-cancer agent is provided which has a higher anti-cancer activity and lower toxicity compared to anti-cancer agents in the related art. In Formulae (1a), (1b), (1c), and (1d), R1 and R2 represent a linear or branched alkyl group, and R4 has a higher priority than R2 as ranked according to RS notation. R3 and R4 represent a hydrogen atom or a linear or branched alkyl group. M represents an atom of a transition metal selected from a group consisting of gold, copper, and silver. X− represents an anion.
