An information processing system comprising: a main device; and an operation device (3, 4) attachable to the main device, wherein the operation device includes a first control circuit (33, 43) for controlling at least a portion of the operation device, a first battery (31, 41) for supplying power to the first control circuit, first power supply means (32, 42) for allowing power supply from the first battery to the first control circuit, and remaining charge amount information transmission means (34, 44, 37, 47) for transmitting, to the main device, remaining charge amount information indicating the amount of charge remaining in the first battery, the main device includes a second control circuit for controlling at least a portion of the main device, a second battery for supplying power to the second control circuit, and second power supply means for allowing power supply from the second battery to the second control circuit, and the second power supply means supplies power from the second battery to the operation device in response to satisfaction of a condition about the amount of remaining charge indicated by the remaining charge amount information transmitted from the remaining charge amount information transmission means, and when power is supplied from the main device to the operation device, the first power supply means supplies power supplied from the main device to the first control circuit, instead of power from the first battery; characterised in that the operation device further includes an operation unit (35, 45), the remaining charge amount information transmission means includes a third communication unit (34,44) for allowing wireless communication with the main device, the main device further includes a fourth communication unit for allowing wireless communication with the operation device, and the remaining charge amount information transmission means transmits operation information indicating an operation performed on the operation unit, and the remaining charge amount information, to the main device by the wireless communication through the third communication unit; wherein when the main device is not supplied with power from an external power source, and power is supplied from the main device to the operation device, the first power supply means supplies power supplied from the main device to the first control circuit, and charges the first battery using power supplied from the main device. a main device; and an operation device (3, 4) attachable to the main device, wherein the operation device includes a first control circuit (33, 43) for controlling at least a portion of the operation device, a first battery (31, 41) for supplying power to the first control circuit, first power supply means (32, 42) for allowing power supply from the first battery to the first control circuit, and remaining charge amount information transmission means (34, 44, 37, 47) for transmitting, to the main device, remaining charge amount information indicating the amount of charge remaining in the first battery, a first control circuit (33, 43) for controlling at least a portion of the operation device, a first battery (31, 41) for supplying power to the first control circuit, first power supply means (32, 42) for allowing power supply from the first battery to the first control circuit, and remaining charge amount information transmission means (34, 44, 37, 47) for transmitting, to the main device, remaining charge amount information indicating the amount of charge remaining in the first battery, the main device includes a second control circuit for controlling at least a portion of the main device, a second battery for supplying power to the second control circuit, and second power supply means for allowing power supply from the second battery to the second control circuit, and a second control circuit for controlling at least a portion of the main device, a second battery for supplying power to the second control circuit, and second power supply means for allowing power supply from the second battery to the second control circuit, and the second power supply means supplies power from the second battery to the operation device in response to satisfaction of a condition about the amount of remaining charge indicated by the remaining charge amount information transmitted from the remaining charge amount information transmission means, and when power is supplied from the main device to the operation device, the first power supply means supplies power supplied from the main device to the first control circuit, instead of power from the first battery; characterised in that the operation device further includes an operation unit (35, 45), the remaining charge amount information transmission means includes a third communication unit (34,44) for allowing wireless communication with the main device, the main device further includes a fourth communication unit for allowing wireless communication with the operation device, and the remaining charge amount information transmission means transmits operation information indicating an operation performed on the operation unit, and the remaining charge amount information, to the main device by the wireless communication through the third communication unit; wherein when the main device is not supplied with power from an external power source, and power is supplied from the main device to the operation device, the first power supply means supplies power supplied from the main device to the first control circuit, and charges the first battery using power supplied from the main device.