PURPOSE: To reduce the consumption of a power source during a power failure by stopping printing operation on detecting the power failure, by storing input data during the power faulure period by a power source for power failure, and by printing the stored data when the power source recovers from the power failure.

CONSTITUTION: Once the power failure of a power source circuit PW occurs, a transistor TR is turned off to inform a CPU of that through an interface circuit VIA. The CPU stops the operation of a printer PRT through the VIA, a flip-flop FF and a printer control circuit CNT. Devices except the PRT are supplied with electric power from a nickel-cadmium battery NID provided for power failures, and input data from a keyboard KB is stored in a nonvolatile memory C-MOSRAM. As the PW recovers from the power failure, the TR is turned on to inform the CPU of that similarly, and the CPU supplies electric power to the PRT through the VIA, etc., to print out the data stored in the C-MOSRAM during the power failure. Thus, the consumption of the power source in case of a power interruption is reduced and even during a power interruption, operations other than printing are performed smoothly.