A POS terminal apparatus is adapted to be connected to an external device. With an application running in the POS terminal apparatus, a device manager determines whether the external device is capable of counting a first value indicating a usage amount of the external device, and if so, the device manager acquires the first value from the external device in response to a first command from the application. A counter retains a second value indicating the usage amount of the external device and notifies the second value to the application in response to a second command from the application. If it is determined that the external device is not capable of counting the first value, the counter virtually counts the second value in accordance with a third command sent from the application to the external device. An updater updates the second value with a third value which is obtained by a fourth command from the application. The first value is retained even when the second value is updated with the third value.