A power converter for an electric vehicle, comprising: a semiconductor module including a power semiconductor element to convert DC power to AC power; a capacitor for smoothing the DC power; a DC-side connecting conductor for transmitting the DC power from the capacitor to the semiconductor module; a first base that is constructed of a thermally conductive member and forms part of a coolant passage through which coolant flows; and a case that defines a housing space for housing the semiconductor module and is provided with an opening portion, wherein: the first base and a second base are forming a partition wall to separate the housing space into two cooling chambers and the semiconductor module is disposed in one chamber at one side of the partition wall and the capacitor is disposed at another side of the partition wall, opposite to the semiconductor module via the partition wall; whereby the semiconductor module and the capacitor are connected with each other via a through-hole in the partition wall, the DC-side connecting conductor extending through the through-hole to electrically connect the capacitor with the semiconductor module; the semiconductor module is provided with a heat transfer plate disposed over the area where the coolant passage is formed, wherein on the heat transfer plate the power semiconductor element is mounted via an insulating substrate; and the heat transfer plate closes an opening of the coolant passage so as to directly contact with the coolant. a semiconductor module including a power semiconductor element to convert DC power to AC power; a capacitor for smoothing the DC power; a DC-side connecting conductor for transmitting the DC power from the capacitor to the semiconductor module; a first base that is constructed of a thermally conductive member and forms part of a coolant passage through which coolant flows; and a case that defines a housing space for housing the semiconductor module and is provided with an opening portion, wherein: the first base and a second base are forming a partition wall to separate the housing space into two cooling chambers and the semiconductor module is disposed in one chamber at one side of the partition wall and the capacitor is disposed at another side of the partition wall, opposite to the semiconductor module via the partition wall; whereby the semiconductor module and the capacitor are connected with each other via a through-hole in the partition wall, the DC-side connecting conductor extending through the through-hole to electrically connect the capacitor with the semiconductor module; the semiconductor module is provided with a heat transfer plate disposed over the area where the coolant passage is formed, wherein on the heat transfer plate the power semiconductor element is mounted via an insulating substrate; and the heat transfer plate closes an opening of the coolant passage so as to directly contact with the coolant. the first base and a second base are forming a partition wall to separate the housing space into two cooling chambers and the semiconductor module is disposed in one chamber at one side of the partition wall and the capacitor is disposed at another side of the partition wall, opposite to the semiconductor module via the partition wall; whereby the semiconductor module and the capacitor are connected with each other via a through-hole in the partition wall, the DC-side connecting conductor extending through the through-hole to electrically connect the capacitor with the semiconductor module; the semiconductor module is provided with a heat transfer plate disposed over the area where the coolant passage is formed, wherein on the heat transfer plate the power semiconductor element is mounted via an insulating substrate; and the heat transfer plate closes an opening of the coolant passage so as to directly contact with the coolant.