An electric power conversion apparatus, comprising: a semiconductor; a main circuit board having a circuit for driving the semiconductor; a casing housing the semiconductor and the main circuit board and having an opening for the flow-in of first cooling air and an upper surface in which an opening portion is provided for the first cooling air to flow out upwardly in use of the electric power conversion apparatus; and a cover above the opening portion of the casing, with a predetermined gap between the cover and the opening portion of the casing, characterised by: a cooling fin on the back of the casing, the cooling fin being for radiating heat from the semiconductor, wherein the cover is provided above both the opening portion of the casing and the cooling fin and has the predetermined gap above the opening portion and the cooling fin; and a cooling fan for forced air cooling of the cooling fin by second cooling air emitted by the fan and passing the cooling fin on the back surface of the casing, wherein the second cooling air from the cooling fin flows upwards and further flows through the gap between the cover and the opening portion of the casing thereby attracting the first cooling air from the opening portion of the casing, and the second cooling air from the cooling fin and the first cooling air from the opening portion of the casing are emitted from an opening portion of the cover. a semiconductor; a main circuit board having a circuit for driving the semiconductor; a casing housing the semiconductor and the main circuit board and having an opening for the flow-in of first cooling air and an upper surface in which an opening portion is provided for the first cooling air to flow out upwardly in use of the electric power conversion apparatus; and a cover above the opening portion of the casing, with a predetermined gap between the cover and the opening portion of the casing, characterised by: a cooling fin on the back of the casing, the cooling fin being for radiating heat from the semiconductor, wherein the cover is provided above both the opening portion of the casing and the cooling fin and has the predetermined gap above the opening portion and the cooling fin; and a cooling fan for forced air cooling of the cooling fin by second cooling air emitted by the fan and passing the cooling fin on the back surface of the casing, wherein the second cooling air from the cooling fin flows upwards and further flows through the gap between the cover and the opening portion of the casing thereby attracting the first cooling air from the opening portion of the casing, and the second cooling air from the cooling fin and the first cooling air from the opening portion of the casing are emitted from an opening portion of the cover. a cooling fin on the back of the casing, the cooling fin being for radiating heat from the semiconductor, wherein the cover is provided above both the opening portion of the casing and the cooling fin and has the predetermined gap above the opening portion and the cooling fin; and a cooling fan for forced air cooling of the cooling fin by second cooling air emitted by the fan and passing the cooling fin on the back surface of the casing, wherein the second cooling air from the cooling fin flows upwards and further flows through the gap between the cover and the opening portion of the casing thereby attracting the first cooling air from the opening portion of the casing, and the second cooling air from the cooling fin and the first cooling air from the opening portion of the casing are emitted from an opening portion of the cover.