To provide a compact vehicular transmission mechanism of a double-drum type dispensing with a large device like a stopper for positioning a shift drum.

A vehicular transmission mechanism is provided with a shift drum turning position determining device comprising: a first shift drum turnably supported in a crankcase; a second shift drum coaxial with the first shift drum and relatively turnably supported on an outer circumference of the first shift drum; star-like members to be engaged provided to the first shift drum and the second shift drum respectively, disposed adjacently each other in the axial direction and formed of recessed parts to be engaged on the outer circumferences; and an engaging member turnably attached the crankcase, having one engaging part engaged with the recessed part of the star-like members to be engaged at a free end side, the engaging part being engaged by straddling both of the star-like members to be engaged fixed to both shift drums.