An electric power supply device supplies electric power to a work machine having an electric power source. The electric power supply device includes a fuel cell different from the electric power source and an electric power connecting section for transferring electric power between the electric power source and the electric power supply device. A control device for controlling the electric power supply device includes a rated capability obtaining section for obtaining the information indicating the rated value of the charge and discharge capability of the electric power source of the work machine in response to the electric power supply device being mounted in the work machine or the electric power supply device being able to utilize the electric power of the electric power source, and an operating condition determining section for determining the operating condition of the electric power supply device based on the rated value.