An ink jet printer comprises a print misalignment detection sensor having a support member for supporting an LED and a PD. The support member is mounted on a cradle holding a printing head, and is open on its cradle-mounting side. The support member has U-shaped light transmitting and receiving grooves respectively forming a light transmitting path from the LED and a light receiving path to the PD. The cradle has light transmitting and receiving path ribs, each with a semicircular recess formed at its end. The two ends of the two light path ribs are respectively inserted into the two light grooves so as to cause the two light paths at the insertion positions to each have a round shape as seen in cross-sectional view. This reduces irregular reflection of light in the two light paths, thereby reducing noise in an output signal of the print misalignment detection sensor.