A paper edge detecting device for a printing apparatus in which a paper margin of a print paper can be detected with high accuracy. A mark printing process is executed upon power-on of a printer, so that a carriage is displaced from an original position H0 to a position corresponding to a set pulse number H.sub.P and stopped at that position to print on the print paper a linear mark M aligned in a direction perpendicular to a displacement direction of the carriage. Thereafter, a mark detection process is executed, and the carriage is displaced from the original position H0 until the mark M is detected by the photosensor. Subsequently, a correction amount generating process is executed to calculate a correction value .alpha. on the basis of the set pulse number H.sub.P and the count value C.sub.P when the mark M is detected by the photosensor.