Abstract: A printing machine such as a rotary offset press includes a printed material monitoring apparatus comprising sensor units for detecting light reflected from a number of unit pixel domains which are divided in a direction perpendicular to a direction in which a print surface of a printed material flows, optical fiber units for transmitting light reflected from the unit pixel domains to the sensor units and a signal processing unit for processing signals transmitted from the sensor units. The sensor units comprise detection sensors each being equipped with a plurality of light reception elements corresponding to the unit pixel domains respectively. The light reception elements are arranged in a plurality of rows of a fixed amount in a direction in which the print surface of the printed material flows to form groups of fixed amount of rows in which the light reception elements are arranged.