Abstract: A gear-shift mechanism for a transmission unit which includes a fork shaft slidably supported in a housing assembly which is selectively shifted from its neutral position to forward and reverse positions, a detent mechanism for selectively retaining the fork shaft in its neutral and shifted positions, a shift head member slidable on the fork shaft, a one-way engagement mechanism for effecting the engagement between the shift head member and the fork shaft during axial displacement of the fork shaft toward and away from its reverse position, and a shift arm member pivotably mounted to the inner wall structure of the housing assembly and linked with the shift head member to retain an idler gear in its neutral position and engage the same with driving and driven gears. The detent mechanism includes a holder member having a slot in its upper portion and a shaft fixed to the upper side portion of the holder member.