To provide a collision sensor excellent in responsiveness and detection sensitivity and easy to construct, while preventing troubles caused by collision with a horizontally articulated robot.

The horizontally articulated robot has a base 15, a first arm 2, a second arm rotating shaft 10, a second arm 3, a third arm rotating shaft 11, and a third arm 4. The robot also has two encoders provided at either two of a motor shaft 5, the second arm rotating shaft 10, and the third arm rotating shaft 11 for detecting their relative rotating angles, and a signal processing circuit 1 for detecting, comparing and monitoring the relative rotating angles upon receiving signals from the two encoders. The two encoders are of the magnetic type and constructed of respective magnetic slit discs 6 and 12 each having a number of slits 21 formed therein and coaxially secured in place, and respective magnetic sensor heads 13 and 14. The signal processing circuit 1 undergoes phase modulation to detect the rotating angles upon receiving two-phase signals of the magnetic sensor heads which are 90 degrees out of phase with each other.