To provide a motor controller which can detect exact speed over the entire speed control range by detecting the speed based on the detection of position and the time required for change of position when it is above a certain speed, and by detecting the speed based on the change of position during sampling time when it is below a certain speed, and to provide its speed detection method.

An encoder 3 outputs time data, i.e. measurements of time required for change of positional data during the sampling time. A speed calculation section 15 defines the positional data as first positional data, generates second positional data by decimating predetermined amount of the first positional data, and generates third positional data by decimating predetermined amount of the second positional data, and then calculates the motor speed based on the second and third positional data.