A communication terminal device having an image memory (21) which does not overflow even if it accepts data during an interrupted condition. When an interruption key (14) on an operation panel (13) is pressed, the communication terminal device enters an interrupt condition. During the interrupt condition, the device receives data through its NCU (19) and modem (20) and stores it in the image memory. If a user inputs no instructions to the device from the operation panel (13) for a predetermined period during the interrupt condition, a printer (16) is caused to print the image stored in the image memory (21). Thus, the image memory has a room for newly coming data, and does not overflow. If the interrupt process needs to use the printer (16), the printing by the printer is stopped, and the interrupt process is given priority.