PURPOSE: An inexpensive titled detector stable to noise and not requiring any adjusting work, wherein a detecting shutter plate is provided at the first group of printing elements, while a photoelectric detector is provided at the second group of printing elements, and the width of slit form holes of the shutter plate is set to be large.

CONSTITUTION: The first and second group 1a, 1b of the printing elements 1 are faced respectively to printing mediums 2a, 2b, and are reciprocated by guiding along a guide shaft 6 in the longitudinal direction. The detecting shutter plate 10 is provided as one body with the first group 1a of the printing elements, while the photoelectric detector 11 is provided as one body with the second group 1b, and the groups 1a, 1b of the printing elements are oppositely moved by oscillating cams 9a, 9b. Since both of the groups are relatively moved and a section indicated by an arrow C is moved by an amount equal to 2 times of the displacement of the printing element groups, the pitch of the slit form holes of the plate 10 can be set to be 2 times of the dot pitch P, a sufficient quantity of light can be processed by the detector 11, and stability of the position detector to noise, disturbing light or the like can be enhanced.