An optical sensor includes: a light emitter; a light receiver; a rod-shaped grasping member in which one of the light emitter and the light receiver is disposed, and which is to be grasped by a hand of a subject; and a clamping member which is adapted to clamp a part of the hand of the subject with the grasping member, another one of the light emitter and the light receiver being disposed in the clamping member.