PURPOSE: To prevent warpage of paper and prevent effectively jam from occurring without making an overall size large and a printing range narrow in a paper jam preventing mechanism for a dot serial type printer.

CONSTITUTION: When a print head 7 is on the left side, a link 12' is driven by operation of a coil spring, a paper guide L10 is dissociated from a platen, and a paper is held by a card holder 2. On the right side, an electromagnetic solenoid 13 is driven so as to hold the paper by a paper guide R11. When the head 7 is on the right side, the paper guide R11 is dissociated and the paper is held by the card holder 2 so as to hold the left thereof by the paper guide L10 in the same manner. When the head 7 is at the center, the paper can be held by the paper guides L10 and R11.