Cover joining structure for outboard engine unit

Fixedly joining sections (31, 32, 33, 51, 52, 53; 141, 142) are provided on opposed joining edges (21e, 41e) of left and right undercover members (21, 41) defining part of an engine space (12) of an outboard engine unit (1). The opposed joining edges are abutted against each other with the joining sections of the left and right undercover members overlapped in face-to-face relation with each other in a front-and-rear direction of the outboard engine unit. Fastener (70, 72; 172) fastens together the overlapped joining sections in the front-and-rear direction, to thereby join together the left and right undercover members. The joining sections may be provided on frame members (23, 24, 30, 43, 44, 50) integrally secured to the respective inner side surfaces (21k, 41k) of resin-made outer wall sections of the left and right undercover members.