To provide a printer detecting an abnormal state of extension and offset of a master.

A printer according to one aspect of the disclosure is a printer for carrying out printing on a print medium using a master 31 wound around a plate cylinder, includes: mark formation means for forming marks 65 for identifying a tip end side of the master 31 and a rear end side of the master 31 in a moving direction of the master 31; reading means for reading the marks 65; and detecting means for detecting whether an abnormal state occurs to extension of the master 31 and offset of the master 31 in accordance with a result of reading the marks 65 read by the reading means.