An electric power converting apparatus configured to be applied to a railway electric car operated in an alternate-current electrified railway section, the electric power converting apparatus comprising: a converter unit (20, 20a, 20b) that converts an alternate current to a direct current; an inverter unit that is connected to an output side of the converter unit (20, 20a, 20b) via a direct-current power source and converts a direct current from the direct-current power source into an alternate current to supply an electric power to an electric motor that drives the railway electric car; and a cooling device (50A, 50B) for cooling a plurality of switching elements included in each of the converter unit (20, 20a, 20b) and the inverter unit, wherein the cooling device (50A, 50B) includes a fin base (40A, 40B) that serves as a switching element attachment plane on which the switching elements are mounted, and a plurality of fins (42A, 42B) arranged on a plane opposite to the switching element attachment plane such that the fins (42A, 42B) are cooled by an airflow caused by traveling of the railway electric car, and the switching elements of the converter unit (20, 20a, 20b) and the switching elements of the inverter unit have different relationships between a heat generation amount and a speed of the railway electric car, wherein the switching elements are cooled by using the airflows caused by the travelling of the railway electric car, the heat generation amount of the switching elements of the converter unit (20, 20a, 20b) is maximal when the speed of the railway electric car is in a speed range that is equal to or higher than 30% to 50% of a maximum speed, charactarised in that a first fin pitch of the fins for the converter unit (20, 20a, 20b) is set to a value with which a cooling performance of the cooling device for the converter unit (20, 20a, 20b) is maximized when the speed of the railway electric car is in a speed range that is equal to or higher than 30%, the heat generation amount of the switching elements of the inverter unit is maximal when the speed of the railway electric car is in a speed range that is equal to or lower than 30% to 50% of the maximum speed, a second fin pitch of the fins for the inverter unit is set to a value with which a cooling performance of the cooling device for the inverter unit is maximized when the speed of the railway electric car is in a speed range that is equal to or lower than 50%, and the first fin pitch is smaller than the second fin pitch. a converter unit (20, 20a, 20b) that converts an alternate current to a direct current; an inverter unit that is connected to an output side of the converter unit (20, 20a, 20b) via a direct-current power source and converts a direct current from the direct-current power source into an alternate current to supply an electric power to an electric motor that drives the railway electric car; and a cooling device (50A, 50B) for cooling a plurality of switching elements included in each of the converter unit (20, 20a, 20b) and the inverter unit, wherein the cooling device (50A, 50B) includes a fin base (40A, 40B) that serves as a switching element attachment plane on which the switching elements are mounted, and a plurality of fins (42A, 42B) arranged on a plane opposite to the switching element attachment plane such that the fins (42A, 42B) are cooled by an airflow caused by traveling of the railway electric car, and the switching elements of the converter unit (20, 20a, 20b) and the switching elements of the inverter unit have different relationships between a heat generation amount and a speed of the railway electric car, wherein the switching elements are cooled by using the airflows caused by the travelling of the railway electric car, the heat generation amount of the switching elements of the converter unit (20, 20a, 20b) is maximal when the speed of the railway electric car is in a speed range that is equal to or higher than 30% to 50% of a maximum speed, charactarised in that a first fin pitch of the fins for the converter unit (20, 20a, 20b) is set to a value with which a cooling performance of the cooling device for the converter unit (20, 20a, 20b) is maximized when the speed of the railway electric car is in a speed range that is equal to or higher than 30%, the heat generation amount of the switching elements of the inverter unit is maximal when the speed of the railway electric car is in a speed range that is equal to or lower than 30% to 50% of the maximum speed, a second fin pitch of the fins for the inverter unit is set to a value with which a cooling performance of the cooling device for the inverter unit is maximized when the speed of the railway electric car is in a speed range that is equal to or lower than 50%, and the first fin pitch is smaller than the second fin pitch.