A power conversion device(100,200) comprising, in a housing: power semiconductor elements; a support case, to which a DC input terminal and an AC output terminal are attached, the DC input terminal and the AC output terminal being connected to the power semiconductor elements, a heat sink, in which a plurality of boss portions are formed on an upper surface thereof, and to which the power semiconductor elements and the support case are attached; a case member having a plurality of leg portions; a smoothing capacitor accommodated in the case member; and a bus bar configured to connect the smoothing capacitor and the DC input terminal, the bus bar being attached to the DC input terminal by a screw, and the plurality of leg portions of the case member being respectively fixed to the plurality of boss portions of the heat sink, characterized in that materials and dimensions of the respective boss portions, the respective leg portions, the support case, and the bus bar are determined so that a value of (αh×Lh+αs×Ls)/(αr×Lr+αb×Lb) falls within a range of from 0.9 to 1.1, where: αh represents a linear expansion coefficient of the respective boss portions; Lh represents a distance from the upper surface of the heat sink to a seat of the respective boss portions; αs represents a linear expansion coefficient of the respective leg portions; Ls represents a length of the respective leg portions extending from the case member to the respective boss portions; αr represents a linear expansion coefficient of the support case; Lr represents a distance from the upper surface of the heat sink to a center of a screw hole, into which the screw is inserted; αb represents a linear expansion coefficient of the bus bar, and; Lb represents a distance from an open end of the case member to the center of the screw hole. power semiconductor elements; a support case, to which a DC input terminal and an AC output terminal are attached, the DC input terminal and the AC output terminal being connected to the power semiconductor elements, a heat sink, in which a plurality of boss portions are formed on an upper surface thereof, and to which the power semiconductor elements and the support case are attached; a case member having a plurality of leg portions; a smoothing capacitor accommodated in the case member; and a bus bar configured to connect the smoothing capacitor and the DC input terminal, the bus bar being attached to the DC input terminal by a screw, and the plurality of leg portions of the case member being respectively fixed to the plurality of boss portions of the heat sink, characterized in that materials and dimensions of the respective boss portions, the respective leg portions, the support case, and the bus bar are determined so that a value of (αh×Lh+αs×Ls)/(αr×Lr+αb×Lb) falls within a range of from 0.9 to 1.1, where: αh represents a linear expansion coefficient of the respective boss portions; Lh represents a distance from the upper surface of the heat sink to a seat of the respective boss portions; αs represents a linear expansion coefficient of the respective leg portions; Ls represents a length of the respective leg portions extending from the case member to the respective boss portions; αr represents a linear expansion coefficient of the support case; Lr represents a distance from the upper surface of the heat sink to a center of a screw hole, into which the screw is inserted; αb represents a linear expansion coefficient of the bus bar, and; Lb represents a distance from an open end of the case member to the center of the screw hole. αh represents a linear expansion coefficient of the respective boss portions; Lh represents a distance from the upper surface of the heat sink to a seat of the respective boss portions; αs represents a linear expansion coefficient of the respective leg portions; Ls represents a length of the respective leg portions extending from the case member to the respective boss portions; αr represents a linear expansion coefficient of the support case; Lr represents a distance from the upper surface of the heat sink to a center of a screw hole, into which the screw is inserted; αb represents a linear expansion coefficient of the bus bar, and; Lb represents a distance from an open end of the case member to the center of the screw hole.