Ink jet printer

A light guide 3 c of the printing shear detection sensor 3 is shaped so that the width of a side cross section when viewed in the conveying direction of a recording paper sheet P indicated by arrow B may be gradually smaller with an increasing degree of change from the top toward the bottom. The light emitted from the light emitting device 3 a is incident on the light guide 3 c from above, passes through the light guide 3 c while being reflected on the side surface of the light guide 3 c, is emitted toward the recording paper sheet P from the bottom of the light guide 3 c, converged and projected on the recording paper sheet P as slit-like light. The light reflected on the recording paper sheet P is incident on the light receiving device 3 b and thus, the printing shear detection pattern formed on the recording paper sheet P can be detected with high accuracy. The light guide 3 c can be manufactured with using an inexpensive molding die at low costs, distinct from the conventional condenser lens.