An offset fin manufacturing method for manufacturing offset fins includes a feeding step of feeding a strip plate, a connection forming step of forming a connection in the strip plate, and an offset fin forming step of bending a planar plate portion located between corresponding two of the connections. In the offset fin forming step, each offset fin is formed such that two of top surface portions, which are located at an upstream end and a downstream end, respectively, of the offset fin in a feed direction of the strip plate, are respectively and directly connected to corresponding two of the connections. One of two of lateral surface portions joined to a corresponding one of the two of the top surface portions is not offset, and thereby a length of the corresponding one of the two of the top surface portions measured along a wave continuation direction is increased.