PURPOSE: To obtain the same printing result without setting up printing starting margin and printing completion margin at every performance of printing by a user.

CONSTITUTION: A printer CPU6 moves a paper detecting sensor 8 up to a physically most right side column printing position from a physically most left side column printing position and detects a paper width and paper position. The printer CPU6 decides a paper size by cling a detected paper width and the paper width stored in a paper width parameter value storing part 3a with each other. The printer CPU6 calculates a difference between a half of a paper width of the decided paper size and a value of a left margin, through a result of which whether or not a printing reference position at the time of setting up of the left margin is the same as a printing reference position of an automatic device is decided. The printer CPU6 caluculates a fresh margin and a printing range is decided. When the decided printing range is held within the paper, the printing is performed by making use of the newly set up left margin.