PURPOSE: To automatically detect a paper size, by providing a sensor detecting the presence/absence of paper for a carriage of a printer and operating the carriage at start.

CONSTITUTION: When a microprocessor 16 receives a printer paper width detection command with a command signal from an external device, an operation command is given to a motor control section 10 to output a paper width detection command signal (a). A paper detection sensor SE provided at a carriage detects the left end of a paper, passes through a vacant part for tractor and detects a paper further, then a programmable counter 14 is controlled and a count output signal (c) is obtained, and a signal (d) being an AND output with both signals (b) and (c) and the command signal (a) is applied to the microprocessor 16 and the number of pulses transmitted to the motor control section 10 from the leading to the trailing is stored in the storage section in a memory 17. This number of pulses is converted into a distance and the paper size can be obtained.