A transmission including a positive clutch configured to selectively connect a gear(41, 51) to a shaft, the gear(41, 51) having first dog teeth(43, 53) formed at an end surface thereof in an axial direction, the gear(41, 53) being disposed around the shaft, wherein the positive clutch comprises: an annular hub connected to the shaft, and having, at an outer peripheral surface thereof, grooves formed in parallel to the shaft; an annular clutch ring having, at an inner peripheral surface thereof, teeth formed in parallel to the shaft, and having, at an end surface thereof, second dog teeth to be meshed with the first dog teeth when the teeth are fitted in the grooves and thereby the clutch ring moves in the axial direction; and a shift device configured to set a position in the axial direction of the clutch ring, each of the first dog teeth(43, 53) has a first surface(46, 56) facing one side in a circumferential direction, and a second surface(47, 57) facing the other side in the circumferential direction, each of the second dog teeth has a third surface opposing the first surface(46, 56), and a fourth surface opposing the second surface(47, 57), the first surface and the third surface are inclined surfaces that are configured such that they generate a thrust that separates the gear(41, 51) and the clutch ring from each other in the axial direction, according to a torque in a direction in which the first surface(46, 56) and the third surface are brought into contact with each other, the second surface(47, 57) and the fourth surface are configured such that they do not cause the gear(41, 51) and the clutch ring to be separated from each other in the axial direction, when the second surface(47, 57) and the fourth surface are brought into contact with each other to transmit a torque, and when gear shift is performed, the shift device is configured to bring the first surface of one said gear into contact with the third surface of the clutch ring, and to bring the second surface of another said gear into contact with the fourth surface of the clutch ring, characterized in that the clutch ring includes an annular ring, and the second dog teeth protrude in the axial direction from an end surface of the ring, and the teeth are formed at inner sides of the second dog teeth, and wherein the second dog teeth include third teeth, and fourth teeth having a length shorter in the axial direction than a length in the axial direction of the third teeth, and the teeth are formed only at the inner sides of the third teeth. an annular hub connected to the shaft, and having, at an outer peripheral surface thereof, grooves formed in parallel to the shaft; an annular clutch ring having, at an inner peripheral surface thereof, teeth formed in parallel to the shaft, and having, at an end surface thereof, second dog teeth to be meshed with the first dog teeth when the teeth are fitted in the grooves and thereby the clutch ring moves in the axial direction; and a shift device configured to set a position in the axial direction of the clutch ring, each of the first dog teeth(43, 53) has a first surface(46, 56) facing one side in a circumferential direction, and a second surface(47, 57) facing the other side in the circumferential direction, each of the second dog teeth has a third surface opposing the first surface(46, 56), and a fourth surface opposing the second surface(47, 57), the first surface and the third surface are inclined surfaces that are configured such that they generate a thrust that separates the gear(41, 51) and the clutch ring from each other in the axial direction, according to a torque in a direction in which the first surface(46, 56) and the third surface are brought into contact with each other, the second surface(47, 57) and the fourth surface are configured such that they do not cause the gear(41, 51) and the clutch ring to be separated from each other in the axial direction, when the second surface(47, 57) and the fourth surface are brought into contact with each other to transmit a torque, and when gear shift is performed, the shift device is configured to bring the first surface of one said gear into contact with the third surface of the clutch ring, and to bring the second surface of another said gear into contact with the fourth surface of the clutch ring, characterized in that the clutch ring includes an annular ring, and the second dog teeth protrude in the axial direction from an end surface of the ring, and the teeth are formed at inner sides of the second dog teeth, and wherein the second dog teeth include third teeth, and fourth teeth having a length shorter in the axial direction than a length in the axial direction of the third teeth, and the teeth are formed only at the inner sides of the third teeth.