A printer includes a casing in which a confronting portion is provided between a platen roller and a thermal line head. The casing has an opening. A guide member provides an inlet passage in the casing and includes a plurality of ribs and a flat surface portion. The ribs are spaced away from each other in a widthwise direction of a sheet. The ribs are in contact with the sheet to guide the sheet. The flat surface portion is positioned at a portion other than the plurality of ribs. An optical sensor is provided to optically detect the sheet. A light shielding member is provided in the casing and protrudes toward the inlet passage at a position closer to the opening than the optical sensor to the opening. The light shielding member has a protruding end in intimate contact with the flat surface portion.