A component mounting part has a width shorter than a distance between guiderails. Backside projected parts protrude in a width direction from the component mounting part at a case-backside end. A width K2 which is a sum of a width of the component mounting part and widths of the backside projected parts is greater than a distance G1 between the guiderails. A clearance is formed between the circuit board and the heat conduction member when the backside projected parts are supported by the guiderails. A distance G3 from an insertion slot to an end of the guiderail on the case backside, is shorter than a distance K4 from an end of the circuit board on the side of the insertion slot to an end of the backside projected part on the side of the insertion slot.