A synchromesh mechanism comprising: a shaft, the shaft being provided with a hub having splines at a radially outer side; a first gear that is freely rotatable on the shaft, the first gear having a first dog clutch gear, and the first gear having a first cone portion with a friction surface projecting toward the hub; a second gear that is freely rotatable on the shaft, the second gear having a second dog clutch gear, and the second gear having a second cone portion with a friction surface projecting toward the hub; a shift sleeve having splines on an inner surface engaged with the splines of the hub to relatively move in an axial direction and engageable with one of the first dog clutch gear and the second dog clutch gear, the splines having chamfers (51a, 51b) at both end sides of the splines; a first synchronizer ring that is arranged between the first cone portion of the first gear and hub, the first synchronizer ring having a frictional surface on an inner surface that is capable of contacting with the first cone portion of the first gear to generate frictional torque, and the first synchronizer ring having splines with chamfers; and a second synchronizer ring that is arranged between the second cone portion of the second gear and the hub, the second synchronizer ring having a frictional surface on an inner surface that is capable of contacting with the second cone portion of the second gear to generate frictional torque, and the second synchronizer ring having splines with chamfers, characterised in that the shift sleeve has a length such that the chamfers of one end side of the splines of the shift sleeve can contact with the chamfers of the splines of the first synchronizer ring and, at the same time, the chamfers of the other end side of the splines of the shift sleeve contact with the chamfers of the splines of the second synchronizer ring. a shaft, the shaft being provided with a hub having splines at a radially outer side; a first gear that is freely rotatable on the shaft, the first gear having a first dog clutch gear, and the first gear having a first cone portion with a friction surface projecting toward the hub; a second gear that is freely rotatable on the shaft, the second gear having a second dog clutch gear, and the second gear having a second cone portion with a friction surface projecting toward the hub; a shift sleeve having splines on an inner surface engaged with the splines of the hub to relatively move in an axial direction and engageable with one of the first dog clutch gear and the second dog clutch gear, the splines having chamfers (51a, 51b) at both end sides of the splines; a first synchronizer ring that is arranged between the first cone portion of the first gear and hub, the first synchronizer ring having a frictional surface on an inner surface that is capable of contacting with the first cone portion of the first gear to generate frictional torque, and the first synchronizer ring having splines with chamfers; and a second synchronizer ring that is arranged between the second cone portion of the second gear and the hub, the second synchronizer ring having a frictional surface on an inner surface that is capable of contacting with the second cone portion of the second gear to generate frictional torque, and the second synchronizer ring having splines with chamfers, characterised in that the shift sleeve has a length such that the chamfers of one end side of the splines of the shift sleeve can contact with the chamfers of the splines of the first synchronizer ring and, at the same time, the chamfers of the other end side of the splines of the shift sleeve contact with the chamfers of the splines of the second synchronizer ring.