An encoder apparatus, comprising: a detection system including a light emitter, a light receiving sensor, and a detector, wherein the detection system is configured to optically detect position information on a scale and to change a relative position thereof with the scale in response to movement of a mover; a signal generator configured to output a signal in response to the movement of the mover; a battery configured to supply at least a part of power consumed by the detection system in accordance with the signal generated by the signal generator; a switcher that is configured to switch between supply and no supply of power to the detection system based on the signal, wherein the switcher is configured to initiate supply of power from the battery to the detection system when a level of the signal generated by the signal generator becomes equal to or higher than a threshold and to stop causing the supply of power from the battery to the detection system when the level of the signal generated by the signal generator becomes lower than the threshold; and an adjuster provided in a power supply path between the switcher and the light emitter and between the switcher and the light receiving sensor that is configured to control operation of the detection system based on the signal, wherein the adjuster is configured to control emission of light to the scale by the light emitter and reception of light by the light receiving sensor; the adjuster is configured to stop causing the supply of power to the light emitter and the light receiving sensor at a predetermined time; whereby the adjuster is configured to set a length of a period during which light is emitted from the light emitter to be shorter than a length of a period during which power is supplied to the detection system based on the electric signal generated by the signal generator; and the detector is configured to perform detection operation of multi-rotation information indicating a number of rotations of the mover at or after the predetermined time based on a detection result of the light receiving sensor. a detection system including a light emitter, a light receiving sensor, and a detector, wherein the detection system is configured to optically detect position information on a scale and to change a relative position thereof with the scale in response to movement of a mover; a light emitter, a light receiving sensor, and a detector, wherein the detection system is configured to optically detect position information on a scale and to change a relative position thereof with the scale in response to movement of a mover; a signal generator configured to output a signal in response to the movement of the mover; a battery configured to supply at least a part of power consumed by the detection system in accordance with the signal generated by the signal generator; a switcher that is configured to switch between supply and no supply of power to the detection system based on the signal, wherein the switcher is configured to initiate supply of power from the battery to the detection system when a level of the signal generated by the signal generator becomes equal to or higher than a threshold and to stop causing the supply of power from the battery to the detection system when the level of the signal generated by the signal generator becomes lower than the threshold; and an adjuster provided in a power supply path between the switcher and the light emitter and between the switcher and the light receiving sensor that is configured to control operation of the detection system based on the signal, wherein the adjuster is configured to control emission of light to the scale by the light emitter and reception of light by the light receiving sensor; the adjuster is configured to stop causing the supply of power to the light emitter and the light receiving sensor at a predetermined time; whereby the adjuster is configured to set a length of a period during which light is emitted from the light emitter to be shorter than a length of a period during which power is supplied to the detection system based on the electric signal generated by the signal generator; and the detector is configured to perform detection operation of multi-rotation information indicating a number of rotations of the mover at or after the predetermined time based on a detection result of the light receiving sensor. the adjuster is configured to control emission of light to the scale by the light emitter and reception of light by the light receiving sensor; the adjuster is configured to stop causing the supply of power to the light emitter and the light receiving sensor at a predetermined time; whereby the adjuster is configured to set a length of a period during which light is emitted from the light emitter to be shorter than a length of a period during which power is supplied to the detection system based on the electric signal generated by the signal generator; and the detector is configured to perform detection operation of multi-rotation information indicating a number of rotations of the mover at or after the predetermined time based on a detection result of the light receiving sensor.