An interlock mechanism includes a shift-and-select shaft a first fork, a second fork, a first inner lever, a second inner lever, an interlock member coaxially arranged around an outer circumference of the shift-and-select shaft while being allowed to rotate together with the shift-and-select shaft in the rotation direction as a unit and not allowed to be displaced in the axial direction, a first engagement portion formed on the first fork, a second engagement portion formed to project from the second fork, a first engaged portion formed to project from the inter lock member and engaged with the first engagement portion while the interlock member is positioned at a second rotation position and a second engaged portion formed on the interlock member and engaged with the second engagement portion while the interlock member is positioned at a first rotation position.

A gearbox of a power unit includes a transmission, a shift drum provided with lead grooves, a shift drum feed mechanism, and a drive side-shift fork and driven side-shift forks respectively provided with engaging portions to be engaged with the lead grooves. At least one lead groove is a common lead groove with which the engaging portion of the drive side-shift fork and the engaging portion of the driven side-shift fork are engaged at an angular interval, which is a positive multiple integral of a feed angle of the shift drum, with respect to a rotation center of the shift drum. The drive side-shift fork and driven side-shift fork, whose engaging portions are engaged with the common lead groove, are guided by the same axially shifted groove portions provided in the common lead groove, and thereby establish speeds of the transmission.

A gear-shifting device for a vehicle is provided and includes a guide body that has a first end portion connected to a control shaft, installed so that a second end portion is rotated at the first end portion as an axis in a rotational displacement that corresponds to a selecting path to perform a selecting operation, and installed to be moved in a substantially straight displacement that corresponds to a shifting path along an axis direction of the first end portion to perform a shifting operation. A guide unit is disposed to distinguish each shifting path on the selecting path in which the second end portion of the guide body is rotated and is configured to guide the second end portion of the guide body to be movable along the shifting path.