A heat exchanger includes a plurality of plate-shaped heat exchanger units and a plurality of flat fins arranged on the units, respectively. Each of the units has a first planar fluid communication chamber for receiving the flat fin. The flat fin has a first end provided with a first recess and a second end opposing to the first end and provided with a second recess. The first recess includes a circular opening and a cutout part. The second recess includes a circular opening and a cutout part. A clearance is defined between an edge of the first end and an inner periphery of the chamber, while another clearance is defined between an edge of the second end and the inner periphery. Consequently, a fluid can flow along the edge of the first end after being introduced into the first planar fluid communication chamber through the first recess.