PURPOSE: To allow one sensor to detect acceleration in two directions and to improve the stability and reliability with simple structure by arraying respective optical fibers for photodetection on the same circumference so that reflected light from a reflecting plate is equal in quantity.

CONSTITUTION: Light from a light emitting element 8 is passed through an optical fiber 1 and a microlens 4 and reflected by the reflecting plate 5, and the reflected light is passed through the lens 4 and optical fibers 21W2n, photodetected by photodetecting elements 91W9n and processed by photodetecting circuits 101W 10n, whose outputs V1WVn proportional to their photodetection levels are supplied to a processing circuit 11, thereby generating outputs Vx and Vy proportional to the orthgonal (x) and (y) components of the acceleration. In this case, the fibers 21W2n are arrayed in a square shape and the real image of the reflected light is formed almost in the center of the fibers 21W2n when the acceleration is zero and the angle θ of the reflecting plate is 0.