An optical encoder including a sensor module including a light emitting member, a light receiving member opposite to the light emitting member to receive the emitted light, and at least one boss extending in a first direction; a shade member located between the light emitting member and the light receiving member and having a shading pattern; a first support supporting the sensor module and including at least one recessed portion engaged with the boss, and a groove connected with the recessed portion and extending in a second direction, wherein the sensor module is attached to the first support by being moved in the second direction while the boss is guided by the groove, and wherein the recessed portion has a depth greater than the height of the boss, and a portion of the groove adjacent to the recessed portion has a depth less than the height of the boss.