An axial fan having a rotatable impeller is arranged above a heat sink with a rotation axis of the impeller substantially coincident with a center axis of a base portion of the heat sink. Radially outer ends of fins arranged on the outer peripheral surface of the base station define an envelope which is provided with a recess therein. The axial fan has a hosing including a surrounding portion surrounding the impeller and arms extending from the surrounding portion. Each arm is provided with an engagement portion engaging with the recess of the heat sink, so that the axial fan is secured to the heat sink. Each arm also has an attachment portion at its lower end. At the attachment portion, an assembly of the axial fan and the heat sink is attached to a motherboard with an object to be cooled mounted thereon.