Abstract: The present invention encompasses an acceleration sensor which is capable of detecting the magnitude or the azimuth of acceleration. For this purpose, a ball 2 is set onto a bottom 1b of a tapered housing. A light emitter 3 is disposed thereunder and a light receiving unit 4 is disposed thereabove. The light receiving unit 4 comprises four light receiving elements 5A-5D which are disposed about the optical axis 10. When acceleration force is exerted on the ball 2 and the ball 2 is moved, light spot 20 of the ball 2 is moved also and output signals produced by receiving elements 5A through 5D are changed, thereby acceleration can be detected.