A connecting structure for medical use includes a male assembly (1) and a female connector (2) having a cylindrical configuration. The male assembly (1) includes a male connector (10) having a cylindrical configuration and a threadedly engageable cylinder (20) rotatably coupled to the male connector (10). A male luer portion (11) of the male connector (10) and a female luer portion (2a) of the female connector (2) are joined by turning the threadedly engageable cylinder (20) in a tightening direction when a female screw (21a) of the threadedly engageable cylinder (20) and an engageable protrusion (2c) of the female connector (2) are threadedly engaged with each other. The connecting structure for medical use further includes a loosening prevention mechanism (30) that prohibits rotation of the threadedly engageable cylinder (20) in a loosening direction with respect to the male connector (10) when the male luer portion (11) and the female luer portion (2a) are joined. The loosening prevention mechanism (30) includes ratchet teeth (31) formed in an inner periphery of the threadedly engageable cylinder (20), an elastic projection (32) formed in the male connector (10) and an engageable claw (33) protruded from a free end of the elastic projection (32) and engageable with the ratchet teeth.