A power conversion device comprising: a flow passage casing in which a flow passage through which a cooling medium flows is formed and an opening portion which is communicated with the flow passage is formed; a power module having a bottomed cylindrical portion in which a power semiconductor element is housed and which is inserted into the flow passage through the opening portion, a flange portion which is formed on an opening of the cylindrical portion and is fixed to the flow passage casing so as to close the opening portion, and a first group of radiator fins (14, 144) which are mounted on an outer peripheral surface of the cylindrical portion with a gap of a predetermined distance formed between the flange portion and the first group of radiator fins; and a second group of radiator fins (14, 144) which are mounted on an outer peripheral surface of the cylindrical portion with a gap of a predetermined distance formed between the flange portion and the second group of radiator fins; characterized by a first flow passage control member (16b, 20b) which is arranged in the gap formed between the flange portion and the first group of radiator fins, and introduces the cooling medium into the first group of radiator fins; a second flow passage control member (16b, 20b) which is arranged opposite to the first flow passage control member with the bottomed cylindrical portion sandwiched therebetween, and introduces the cooling medium into the second group of radiator fins; and a seal member having a ring portion which connects the first flow passage control member and the second flow passage control member; wherein the seal member is mounted on the bottomed cylindrical portion; and wherein power is converted into AC power from DC power or into DC power from AC power by a switching operation of the power semiconductor element. a flow passage casing in which a flow passage through which a cooling medium flows is formed and an opening portion which is communicated with the flow passage is formed; a power module having a bottomed cylindrical portion in which a power semiconductor element is housed and which is inserted into the flow passage through the opening portion, a flange portion which is formed on an opening of the cylindrical portion and is fixed to the flow passage casing so as to close the opening portion, and a first group of radiator fins (14, 144) which are mounted on an outer peripheral surface of the cylindrical portion with a gap of a predetermined distance formed between the flange portion and the first group of radiator fins; and a second group of radiator fins (14, 144) which are mounted on an outer peripheral surface of the cylindrical portion with a gap of a predetermined distance formed between the flange portion and the second group of radiator fins; characterized by a first flow passage control member (16b, 20b) which is arranged in the gap formed between the flange portion and the first group of radiator fins, and introduces the cooling medium into the first group of radiator fins; a second flow passage control member (16b, 20b) which is arranged opposite to the first flow passage control member with the bottomed cylindrical portion sandwiched therebetween, and introduces the cooling medium into the second group of radiator fins; and a seal member having a ring portion which connects the first flow passage control member and the second flow passage control member; wherein the seal member is mounted on the bottomed cylindrical portion; and wherein power is converted into AC power from DC power or into DC power from AC power by a switching operation of the power semiconductor element.