PURPOSE: To detect the magnitude and the direction of oscillation, by irradiating the light to photodetectors, which are arranged one-dimensionally, through an optical mask, where the pattern which changes the incident light in accordance with the oscillation input is formed.

CONSTITUTION: When an oscillation is applied to a casell, an oscillation system mass 12 is moved relatively to the case 11, and the pattern of the light of a light source 2 which is transmitted through an optical mask 13 to reach a photodetector array 14a of a photo detecting array circuit 14 is changed. The optical mask 13 consists of a light-transmissive part 13a and a light interceptive part 13b, and a fan-shaped pattern formed with a line (r), which is inclined at 45° to a direction X of the one-dimensional arrangement of the photodetector array 14a, and a line (s) which intersects the line (r) at 90° is used in the light interceptive part 13b. Thus, the variation to the pendulum motion at the intersection among the line (X), the line (r), and the line (s) is indicated as the sum and the difference between the relative variation in the X direction and the relative variation in the Y direction, and X and Y two-dimensional oscillation output is taken out.