Provided is an optional unit configured to be connected, in series, to a power supply apparatus for supplying drive electricity, the optional unit including: a receiving unit which receives the electricity supplied from the power supply apparatus or an optional unit in a higher connection level; a supply unit which supplies the electricity to a receiving unit of an optional unit in a lower connection level; and an operation unit which operates based on the electricity received by the receiving unit. In accordance with a voltage of the supplied electricity, based on which the operation unit of the optional unit in the higher level operates, exceeding a predetermined threshold value, the operation unit of the optional unit in the lower level below the optional unit in the higher level starts to receive the supplied electricity.