To sustain normal print processing by enabling the function of print spool unit through a switch when the power supply voltage of a printer unit drops below a specified level and storing a print data in the print spool unit.

After a stored data is provided with the content at an address designated by a print start pointer and outputted, the print start pointer is advanced. When the address matches a space area pointer, a decision is made that the stored data is outputted entirely and a spool unit 9 can operate as an FIFO. When the power supply voltage of a printer unit drops to disable printing, the spool unit 9 is enabled to store a print data in the HT body and a decision can be made on the application side that print operation has ended normally.