Since the ferromagnetism in ternary chalcopyrite semiconductor CdGeP2:Mn was first reported by Medvedkin et al., several ferromagnetic ternary compounds such as II-IV-P2 and II-IV-As2 doped with Mn have been exploited from the viewpoint of experimental and theoretical aspects. Almost of these compounds have been synthesized in forms of bulk II-IV-V2 chalcopyrite crystals. In this article, we review recent experimental results of diluted ferromagnetic ZnSnAs2:Mn thin films among various ternary ferromagnetic compounds exhibiting room temperature ferromagnetism. We expect that ZnSnAs2:Mn epitaxial thin films is a promising candidate as a ferromagnetic building block in InP-based semiconductor spintronics, although the origin of ferromagnetism is not yet clearly understood. The state of development of ZnSnAs2:Mn thin films is still at a very early stage, and basic information on the ZnSnAs2:Mn is required to elucidate the origin of the ferromagnetism.