A collision detecting method for quickly securely detecting a collision between a driven body driven by a servomotor and an obstacle by using an observer, thereby preventing machine components from be damaged by the collision. An estimated disturbance torque (y) is obtained in accordance with a torque command (I) and an actual motor speed (.theta.) by means of the observer (50) formed of blocks (S1 to 54) having transfer functions (Kt/J, K3, K4/S, 1/S), which depend individually on a torque constant, inertia, and parameters corresponding thereto, and a multiplier (61) having a transfer function (J.multidot.A) equivalent to the product of the inertia and a unit system conversion constant. The drive of the servomotor is stopped before any of the components of the machine is broken, in response to an alarm delivered from a comparator (62) when the estimated disturbance torque exceeds a threshold value (Ts) which is set at a value smaller than servomotor output torques corresponding to the breaking limits of the machine components.