A heatsink to be mounted on a circuit board including a plurality of electronic parts is constituted of a conductive and rectangular plate-shaped member, and mounted on the circuit board such that a main surface of the heatsink blocks an airflow generated on the circuit board, the heatsink being electrically grounded. The main surface includes a contacting portion disposed in contact with the circuit board and an isolated portion separated from the circuit board, the isolated portion being cut into two parts along a straight line extending in a direction away from the circuit board. The two parts are each bent such that an end portion on a side of the straight line is oriented to a downstream side of the airflow, so that an opening is defined between the respective end portions of the two parts on the side of the straight line.