PURPOSE: To enable highly accurate fixed dimension machining having no fluctuation of finished dimension by producing a smoothing signal from continual measurement and performing fixed dimension control on the basis of said signal.

CONSTITUTION: Grinding and measuring of work are repeated alternatively and the continual measurements are employed for machining control. Here the continual measurement signals are fed from a meter 1 to a changeover circuit 10 to produce step signals variable continuously. Said signals are stored alternatively in the first and second memories 11, 12 to obtain the absolute difference of the levels stored in said memories 11, 12 through subtractor 15. The subtracted level is integrated by third integrator 16 then subtracted in operating circuit 18 from the step signal variable continuously to be fed from the changeover circuit 10 thus to produce a smoothing signal which is employed for machining control of work.