A semiconductor module cooler that supplies a cooling medium to a cooling medium jacket from the outside and cools one or a plurality of semiconductor elements which are provided on an outer surface of the semiconductor module cooler and are thermally connected to the cooling medium jacket through a heat sink, wherein the cooling medium jacket has a flat rectangular parallelepiped shape, and wherein the cooling medium jacket includes: a cooling fin cooling room that includes an opening into which cooling fins are inserted, wherein the fins are formed on a surface of the heat sink opposite to a surface to which the semiconductor elements are connected and the cooling fin cooling room cools the cooling fins; a cooling medium introduction port from which the cooling medium is introduced; a cooling medium diffusion room that diffuses the cooling medium introduced from the cooling medium introduction port and supplies the cooling medium to the cooling fin cooling room; a cooling medium diffusion wall that is provided in the cooling medium diffusion room and over which the cooling medium diffused by the cooling medium diffusion room is introduced to the cooling fin cooling room, wherein a surface of the cooling medium diffusion wall facing the cooling medium introduction port has a vertical surface, wherein the cooling medium diffusion room is formed so as to be widened from the cooling medium introduction port to the cooling medium diffusion wall, and wherein a height of the cooling medium diffusion wall is equal to or higher than that of an upper end of the cooling medium introduction port; a cooling medium discharge port through which the cooling medium is discharged to the outside; and a cooling medium convergence room that is provided between the cooling fin cooling room and the cooling medium discharge port, wherein the cooling medium jacket further includes a cooling medium inlet that is formed in a bottom of a central portion of one long side of the cooling medium jacket, wherein the semiconductor module cooler is arranged such that the cooling medium introduced from the cooling medium inlet flows from the cooling medium introduction port to the cooling medium diffusion room. a cooling fin cooling room that includes an opening into which cooling fins are inserted, wherein the fins are formed on a surface of the heat sink opposite to a surface to which the semiconductor elements are connected and the cooling fin cooling room cools the cooling fins; a cooling medium introduction port from which the cooling medium is introduced; a cooling medium diffusion room that diffuses the cooling medium introduced from the cooling medium introduction port and supplies the cooling medium to the cooling fin cooling room; a cooling medium diffusion wall that is provided in the cooling medium diffusion room and over which the cooling medium diffused by the cooling medium diffusion room is introduced to the cooling fin cooling room, wherein a surface of the cooling medium diffusion wall facing the cooling medium introduction port has a vertical surface, wherein the cooling medium diffusion room is formed so as to be widened from the cooling medium introduction port to the cooling medium diffusion wall, and wherein a height of the cooling medium diffusion wall is equal to or higher than that of an upper end of the cooling medium introduction port; wherein the cooling medium diffusion room is formed so as to be widened from the cooling medium introduction port to the cooling medium diffusion wall, and wherein a height of the cooling medium diffusion wall is equal to or higher than that of an upper end of the cooling medium introduction port; a cooling medium discharge port through which the cooling medium is discharged to the outside; and a cooling medium convergence room that is provided between the cooling fin cooling room and the cooling medium discharge port, wherein the cooling medium jacket further includes a cooling medium inlet that is formed in a bottom of a central portion of one long side of the cooling medium jacket, wherein the semiconductor module cooler is arranged such that the cooling medium introduced from the cooling medium inlet flows from the cooling medium introduction port to the cooling medium diffusion room.