A brake structure for a wheel (W) rotating device is disclosed, which comprises a motor (1) provided in a wheel (W) and is driven for rotating the wheel (W), and a braking mechanism (3) for actuating brake to brake the wheel (W). The motor (1) includes: a motor housing (11); a stator (12) positioned in and fixed to the motor housing (11); and a rotor (13) positioned in the motor housing (11) and facing to the stator (12). The braking mechanism (3) includes: a brake rotor (35) to rotate with the wheel (W); frictional members (33b,34b) to be in contact with the brake rotor (35) for generation of braking force; a pressing force generating unit (32) for generating pressing force of the frictional members (33b,34b) so that the frictional members (33b,34b) are urged to and pressed against the brake rotor (35) by supplying brake fluid through a brake fluid passage (111a) to transmit fluid pressure; and a housing (31) for the pressing force generating unit (32). The brake fluid passage (111a) is formed inside a wall of the motor housing (11), and is connected to a brake fluid supply port (31a) provided in the housing (31) for the pressing force generating unit (32).