To improve heat exchange efficiency by eliminating a damage due to freezing of a zigzag pipe upon draining, and mounting many fins on the pipe of a straight section in a heat exchanger at a distance with a small pitch axially of the pipe of the straight section with ease.

Many vertically expandable fin members 13 are mounted over a pipe 12 of a straight section axially thereof with a small pitch at a distance, and a zigzag pipe 10 is installed hierarchically in a direct/alternating flow type cooling tower at a predetermined distance in the state where a flat plane involving the zigzag pipe 10 is inclined downward at the angle of about 2 to 3 degree with respect to a horizontal plane. The fin member 13 comprises a rectangular plate made of a synthetic resin flattened over the entire thereof, and through-holes, into and with which the pipe 12 of the straight section is inserted and brought into close contact, are formed in the rectangular plate in an equal interval with a predetermined pitch. Further, there is formed a slit section extendable radially of the pipe 12 of the straight section between the adjacent insertion holes in the assembled state to the zigzag pipe 10.