PURPOSE: To reduce the width of a sensor unit and to reduce the size of an electrooptical sensor by a constitution, wherein an optical group comprises an optical means for focusing and retracting the light beam emitted from a light emitter.

CONSTITUTION: An optical group comprises two beam focus requlators, including two optical axes x, x' of a light emitter 3 and a light receiver, focus lenses 10, 12, and light-retracting parts 11, 13, wherein the light beam emitted from the light emitter 3 and reflected on a reflective graduation 2 advances in parallel with two optical axes. A dispersing light beam emitted from the light emitter 3 is converted through a lens 10 into a parallel light beam 17 centered on the optical axis. A light refractive plane 11 bends parallel light beams 17 which are then collected on the reflective graduation 2 of a scale 1. A light refractive plane 13 bends parallel light beams reflected on the reflective graduation 2 and passing through a focus plate 14, before being converted into a light beam 18 advancing, while being centered on the optical axis of a light receiver 4. The focus lens 12 focuses the reflected parallel light beams 18 to the light receiver 4. Furthermore, an air layer 19 of specified thickness for determining the etching pitch of the scale 1 and the focus plate 14 is interposed between them.