A semiconductor device that has a semiconductor module and a cooler for cooling a power semiconductor element mounted on a substrate in the semiconductor module, the semiconductor device comprising at least: a heat dissipation base; cooling fins, which are a cluster of a plurality of pin members or blade members, the cluster having an approximately rectangular cuboid contour, and which are provided on a first principal surface of the heat dissipation base; a cooler which is fixed to the heat dissipation base, and has a cooling unit accommodating the cooling fins therein and a coolant inlet and a coolant outlet that are formed at respective ends of the cooling unit to face each other in a longitudinal direction; a first substrate which is bonded, in the vicinity of the coolant inlet, to a second principal surface of the heat dissipation base such that the position of the first substrate corresponds to the position of the cooling fins; and a second substrate which is disposed adjacent to the first substrate and bonded, in the vicinity of the coolant outlet, to the second principal surface of the heat dissipation base, wherein the cooling unit has: a first header part which has a first bottom surface disposed between the coolant inlet and an end portion of the first substrate on the coolant outlet side and inclined toward a bottom plane of the cooling fins so that a coolant supplied from the coolant inlet flows toward the cooling fins; and a second header part which has a second bottom surface that is inclined from an end portion of the bottom plane of the cooling fins on the coolant outlet side so that the coolant discharged from the cooling fins flows to the coolant outlet, wherein the semiconductor device further comprises a third bottom surface which is stretched out between the first bottom surface and the second bottom surface and parallel to the bottom plane of the cooling fins, wherein preferably an inclination angle of the first bottom surface with respect to the bottom plane of the cooling fins is smaller than an inclination angle of the second bottom surface, and the semiconductor device is characterized in that a top portion in which the first bottom surface and the third bottom surface intersect, is located between a lower part of a center of the first substrate and a lower part of an end portion of the first substrate on the coolant outlet side. a heat dissipation base; cooling fins, which are a cluster of a plurality of pin members or blade members, the cluster having an approximately rectangular cuboid contour, and which are provided on a first principal surface of the heat dissipation base; a cooler which is fixed to the heat dissipation base, and has a cooling unit accommodating the cooling fins therein and a coolant inlet and a coolant outlet that are formed at respective ends of the cooling unit to face each other in a longitudinal direction; a first substrate which is bonded, in the vicinity of the coolant inlet, to a second principal surface of the heat dissipation base such that the position of the first substrate corresponds to the position of the cooling fins; and a second substrate which is disposed adjacent to the first substrate and bonded, in the vicinity of the coolant outlet, to the second principal surface of the heat dissipation base, wherein a first header part which has a first bottom surface disposed between the coolant inlet and an end portion of the first substrate on the coolant outlet side and inclined toward a bottom plane of the cooling fins so that a coolant supplied from the coolant inlet flows toward the cooling fins; and a second header part which has a second bottom surface that is inclined from an end portion of the bottom plane of the cooling fins on the coolant outlet side so that the coolant discharged from the cooling fins flows to the coolant outlet, wherein the semiconductor device further comprises a third bottom surface which is stretched out between the first bottom surface and the second bottom surface and parallel to the bottom plane of the cooling fins, wherein preferably an inclination angle of the first bottom surface with respect to the bottom plane of the cooling fins is smaller than an inclination angle of the second bottom surface, and the semiconductor device is characterized in that a top portion in which the first bottom surface and the third bottom surface intersect, is located between a lower part of a center of the first substrate and a lower part of an end portion of the first substrate on the coolant outlet side.