A method of assembling a differential gear set includes installing a first bushing into a carrier, installing a second bushing into the carrier, and inserting a first planet gear into an interior portion of the carrier. The external power source apparatus switches the control signal from a non-oscillation state to an oscillation state when the preparation for power supply is completed. The billing data may be presented in such a way that the customer may input the billing data directly into its client billing system. The second part is laterally offset from the first part, and the second part is approximately parallel to the longitudinal direction. From the first time to the second time, a regulated output voltage is maintained at about a word line operation voltage such as a read voltage. The control circuit generates the control signal to increase the output signal from the light intensity detector based on the light intensity monitor signal and control signal monitor signal.