A reverse inhibitor mechanism for a transmission of an all terrain vehicle, in which a shift drum is jounalled to a side wall opposite the transmission case so as to be rotatably supported thereon, and a reverse inhibitor arm is detachably engaged with the reverse inhibitor groove formed in the outer circumferential surface of the shift drum to regulate a reverse setting. A reverse inhibitor shaft having the reverse inhibitor arm supported at one end thereof is positioned within the transmission case. The other end of the reverse inhibitor shaft is rotatably journaled to one of the opposite side walls of the transmission case from the inside of the transmission case. A distal end of the reverse inhibitor arm which swings by the rotation of the reverse inhibitor shaft is brought into engagement with the reverse inhibitor groove during assembly of the reverse inhibitor mechanism.