A cam device comprising: a drive-side cam; and a driven-side cam; the drive-side cam having a drive-side cam surface provided on one side surface in the axial direction, and the drive-side cam surface comprising a flat shaped drive-side basal surface, and plural drive-side convex sections provided to protrude toward the one side in the axial direction from plural locations in the circumferential direction of the drive-side basal surface; the driven-side cam having a driven-side cam surface provided on the other side surface in the axial direction, and the driven-side cam surface comprising a flat shaped driven-side basal surface, and plural driven-side convex sections provided to protrude toward the other side in the axial direction from plural locations in the circumferential direction of the driven-side basal surface; and by causing the drive-side cam to rotate relative to the driven-side cam, the cam device making it possible to switch from a locked state in which the dimension in the axial direction is increased by the tip-end surfaces of the drive-side convex sections coming in contact with the tip-end surfaces of the driven-side convex sections, and an unlocked state in which the dimension in the axial direction is decreased by arranging the drive-side convex sections and the driven-side convex sections in an alternating manner in the circumferential direction; and the drive-side cam has a drive-side marking formed at one or plural locations in the circumferential direction of the outer-circumferential surface of the drive-side cam, and the driven-side cam has a driven-side marking formed in portions of the outer-circumferential surface of the driven-side cam such that the phase of the driven-side marking in the circumferential direction in the locked state coincides with the phase of the drive-side marking in the circumferential direction, characterized in that a second marking is formed in a portion of the outer circumferential surface of either one of the drive-side cam and the driven-side cam such that the phase of the second marking in the circumferential direction in the unlocked state coincides with the phase of the marking that is formed in the outer-circumferential surface of the other cam of the drive-side cam and the driven-side cam. a drive-side cam; and a driven-side cam; the drive-side cam having a drive-side cam surface provided on one side surface in the axial direction, and the drive-side cam surface comprising a flat shaped drive-side basal surface, and plural drive-side convex sections provided to protrude toward the one side in the axial direction from plural locations in the circumferential direction of the drive-side basal surface; the driven-side cam having a driven-side cam surface provided on the other side surface in the axial direction, and the driven-side cam surface comprising a flat shaped driven-side basal surface, and plural driven-side convex sections provided to protrude toward the other side in the axial direction from plural locations in the circumferential direction of the driven-side basal surface; and by causing the drive-side cam to rotate relative to the driven-side cam, the cam device making it possible to switch from a locked state in which the dimension in the axial direction is increased by the tip-end surfaces of the drive-side convex sections coming in contact with the tip-end surfaces of the driven-side convex sections, and an unlocked state in which the dimension in the axial direction is decreased by arranging the drive-side convex sections and the driven-side convex sections in an alternating manner in the circumferential direction; and the drive-side cam has a drive-side marking formed at one or plural locations in the circumferential direction of the outer-circumferential surface of the drive-side cam, and the driven-side cam has a driven-side marking formed in portions of the outer-circumferential surface of the driven-side cam such that the phase of the driven-side marking in the circumferential direction in the locked state coincides with the phase of the drive-side marking in the circumferential direction, characterized in that a second marking is formed in a portion of the outer circumferential surface of either one of the drive-side cam and the driven-side cam such that the phase of the second marking in the circumferential direction in the unlocked state coincides with the phase of the marking that is formed in the outer-circumferential surface of the other cam of the drive-side cam and the driven-side cam.