A constant-mesh transmission having a synchronizing mechanism, the synchronizing mechanism including a fifth gear which acts as a shifter so as to rotate integrally with a counter shift and movably in the axial direction, a recess formed in a first gear, and a synchronizer ring. A first connection spline formed in the outer circumferential wall face of the recess and a first synchronization spline formed in the outer circumferential face of the synchronizer ring have a predetermined overlap portion, whereby they overlap partly in the axial direction. In the fifth gear, a second connection spline meshes with the first connection spline and a second synchronization spline presses the first synchronization spline in the axial direction to make the synchronizer ring and the first gear come into frictional engagement with each other.