Coupling members (50) are inserted into through-holes (9 and 10) in links (2) of a chain (1). The coupling member (50) includes first and second pins (3 and 4). Pairs of end portions (14 and 15, 27 and 28) of the first and second pins (3 and 4) with respect to an orthogonal direction (V) orthogonal to both a chain travel direction (X) and a chain width direction (W) are received by receiving portions (38 and 39, 40 and 41) provided to the corresponding through-holes (9 and 10) in the corresponding links (2). At least one of the end portions (14, 15, 27, and 28) includes a curved surface portion (14, 15, 32a, or 32a) received by the corresponding receiving portion (38, 39, 40, or 41). The curved surface portion (14, 15, 32a, or 32a) includes an apex portion (20, 22, 33, or 33) with respect to the orthogonal direction (V), and the curvature radius of the curved surface portion (14, 15, 32a, or 32a) is increased continuously or step by step as headed toward the apex portion (20, 22, 33, or 33).