A shift mechanism configured to intermittently rotate a shift cam (110,110B) in response to transmission operation to hold the shift cam (110,110B) in each phase from a neutral phase corresponding to a neutral position to a maximum speed gear phase corresponding to a maximum speed gear position through a first gear phase corresponding to a first speed gear position, the shift mechanism comprising: an index cam (130,130A,130B) that is fixed to one end of the shift cam (110,110B) and configured to rotate integrally with the shift cam (110,110B) to position the shift cam (110,110B) in each of the phases; a drive section (84,85) configured to rotate the index cam (130,130A,130B); and a cam phase holding portion (140,450) configured to hold the rotating shift cam (110,110B) in each of the phases, wherein the index cam (130,130A,130B) comprises concave portions (132,132A) formed on a periphery portion in order of neutral to a maximum speed gear that is a top gear through a first gear in a circumferential direction corresponding to each of the phases, the concave portions (132,132A) being engaged with the cam phase holding portion (140,450) to converge and hold the shift cam (110,110B) in a predetermined phase, characterized in that the concave portions (132,132A) are disposed on the periphery portion of the index cam (130,130A,130B) so that a rotation angle between the concave portions (132,132A) respectively corresponding to the neutral and the first gear is the same as a rotation angle between the concave portions (132,132A) respectively corresponding to the first gear and a second gear, and an angle between the concave portions (132,132A) respectively corresponding to the maximum speed and the neutral is smaller than the rotation angle between the concave portions (132,132A) respectively corresponding to the neutral and the first gear. an index cam (130,130A,130B) that is fixed to one end of the shift cam (110,110B) and configured to rotate integrally with the shift cam (110,110B) to position the shift cam (110,110B) in each of the phases; a drive section (84,85) configured to rotate the index cam (130,130A,130B); and a cam phase holding portion (140,450) configured to hold the rotating shift cam (110,110B) in each of the phases, wherein the index cam (130,130A,130B) comprises concave portions (132,132A) formed on a periphery portion in order of neutral to a maximum speed gear that is a top gear through a first gear in a circumferential direction corresponding to each of the phases, the concave portions (132,132A) being engaged with the cam phase holding portion (140,450) to converge and hold the shift cam (110,110B) in a predetermined phase, characterized in that the concave portions (132,132A) are disposed on the periphery portion of the index cam (130,130A,130B) so that a rotation angle between the concave portions (132,132A) respectively corresponding to the neutral and the first gear is the same as a rotation angle between the concave portions (132,132A) respectively corresponding to the first gear and a second gear, and an angle between the concave portions (132,132A) respectively corresponding to the maximum speed and the neutral is smaller than the rotation angle between the concave portions (132,132A) respectively corresponding to the neutral and the first gear.