1. A method of detecting the speed of a motor, the output shaft of which is connected to a pulse generator for generating pulse signals depending on the angular displacement of the motor, the method comprising the steps of (a) setting an initial value which is used to estimate the speed and an estimated value of the variable representing the torque disturbance both at zero, (b) estimating the speed at the time when a sampling pulse is generated, by integrating the sum of the variable representing the motor torque and the estimated value of the variable representing the torque disturbance with the prior initial value as an initial value, each time the sampling pulse is generated, until the pulse generator generates a pulse signal, (c) calculating the variable representing the torque disturbance when the pulse signal is generated, by using the time period between the time step (a) is executed and the time the pulse signal is generated and the estimated values of the speed calculated at step (b) between this time period, (d) calculating the real speed at the time when the pulse signal is generated, by compensating the speed estimated just before this time, using the amount of compensation for the torque disturbance which is the difference between the variable representing the torque disturbance and the estimated value of the variable representing the torque disturbance, (e) replacing the initial value which is used to estimate the speed by the real speed calculated at step (d), and replacing the estimated value of the variable representing the torque disturbance by the prior variable representing the torque disturbance calculated at step (c), (f) estimating the speed at the time when the sampling pulse is generated, by integrating the sum of the variable representing the motor torque and the estimated value of the variable representing the torque disturbance with the initial value replaced at step (e) as an initial value, each time the sampling pulse is generated, until the pulse generator generates a pulse signal, (g) calculating the variable representing the torque disturbance when the pulse signal is generated, by using the time period between the time the preceding pulse signal is generated and the time the present pulse signal is generated and the estimated values of the speed calculated at step (f) between this time period, (h) calculating the real speed at the time when the pulse signal is generated, by compensating the speed estimated just before this time, using the amount of compensation for the torque disturbance which is the difference between the variable representing the torque disturbance and the estimated value of the variable representing the torque disturbance, (i) replacing the initial value which is used to estimate the speed by the real speed calculated at step (h), and replacing the estimated value of the variable representing torque disturbance by the prior variable representing the torque disturbance calculated at step (g), (j) estimating the speed at the time when the sampling pulse is generated, by integrating the sum of the variable representing the torque disturbance with the initial value replaced at step (i) as an initial value, each time the sampling pulse is generated, until the pulse generator generates a pulse signal, and (k) repeating steps (g)-(j).