A power conversion device comprising: a power module unit which includes power semiconductor modules (300U, 300V, 300W) in each of which power semiconductor devices (328, 330, 156, 166) for converting DC current into AC current are stored in a module case in a bottomed tubular shape and DC terminals, an AC terminal and signal terminals are extracted from an extracting part of the module case, and a first coolant channel body which functions as a cooler for cooling down the power semiconductor modules mounted thereon, and the power semiconductor modules (300U, 300V, 300W) are inserted into a coolant channel of the first coolant channel body so that the extracting parts of the module case are arranged at the face of the first coolant channel body facing the prescribed space, wherein the power conversion device further comprises: a housing having a base on which the first cooling channel body is arranged in parallel across a prescribed space and openings formed in areas of the base facing the prescribed space, a capacitor unit which includes a capacitor bus bar, a plurality of capacitor elements which are connected to the capacitor bus bar in parallel, the DC terminals (157, 158) being connected with the capacitor bus bar at positions facing the openings (100A - 100C), characterized in that the capacitor unit includes a second coolant channel body, which functions as a cooler for cooling down the plurality of capacitor elements fixed thereto and for cooling down the capacitor bus bar, and the power module is arranged in parallel with the capacitor unit, and the capacitor bus bar is extracted from the second coolant channel body to the prescribed space, the first and second cooling channel bodies (12A, 12B) are arranged in parallel across the prescribed space of the base, a first circuit board which is arranged over the power module unit, a driver circuit which is mounted on the first circuit board, a second circuit board which is arranged over the second coolant channel body, and a control circuit which is mounted on the second circuit board. a power module unit which includes power semiconductor modules (300U, 300V, 300W) in each of which power semiconductor devices (328, 330, 156, 166) for converting DC current into AC current are stored in a module case in a bottomed tubular shape and DC terminals, an AC terminal and signal terminals are extracted from an extracting part of the module case, and a first coolant channel body which functions as a cooler for cooling down the power semiconductor modules mounted thereon, and the power semiconductor modules (300U, 300V, 300W) are inserted into a coolant channel of the first coolant channel body so that the extracting parts of the module case are arranged at the face of the first coolant channel body facing the prescribed space, wherein the power conversion device further comprises: a housing having a base on which the first cooling channel body is arranged in parallel across a prescribed space and openings formed in areas of the base facing the prescribed space, a capacitor unit which includes a capacitor bus bar, a plurality of capacitor elements which are connected to the capacitor bus bar in parallel, the DC terminals (157, 158) being connected with the capacitor bus bar at positions facing the openings (100A - 100C), characterized in that the capacitor unit includes a second coolant channel body, which functions as a cooler for cooling down the plurality of capacitor elements fixed thereto and for cooling down the capacitor bus bar, and the power module is arranged in parallel with the capacitor unit, and the capacitor bus bar is extracted from the second coolant channel body to the prescribed space, the first and second cooling channel bodies (12A, 12B) are arranged in parallel across the prescribed space of the base, a first circuit board which is arranged over the power module unit, a driver circuit which is mounted on the first circuit board, a second circuit board which is arranged over the second coolant channel body, and a control circuit which is mounted on the second circuit board. a housing having a base on which the first cooling channel body is arranged in parallel across a prescribed space and openings formed in areas of the base facing the prescribed space, a capacitor unit which includes a capacitor bus bar, a plurality of capacitor elements which are connected to the capacitor bus bar in parallel, the DC terminals (157, 158) being connected with the capacitor bus bar at positions facing the openings (100A - 100C), characterized in that the capacitor unit includes a second coolant channel body, which functions as a cooler for cooling down the plurality of capacitor elements fixed thereto and for cooling down the capacitor bus bar, and the power module is arranged in parallel with the capacitor unit, and the capacitor bus bar is extracted from the second coolant channel body to the prescribed space, the first and second cooling channel bodies (12A, 12B) are arranged in parallel across the prescribed space of the base, a first circuit board which is arranged over the power module unit, a driver circuit which is mounted on the first circuit board, a second circuit board which is arranged over the second coolant channel body, and a control circuit which is mounted on the second circuit board.