PURPOSE: To improve an accuracy of size of a workpiece, prevent an excessive margin of it and provide a small-sized and light weight of the double head plainer by a method wherein a contact finger is contacted with a machining surface of a workpiece so as to sense a size of premachined workpiece and the sensed signal is sent to a control unit.

CONSTITUTION: There are provided the first and second grinder stones 15, 16 for use in plain machining of a workpieces 1 and a holding means for fast or loose holding the workpiece 1, and a workpiece 1 between the grinder stones 15, 16 in reference to a workpiece feeding speed signal sent from the control unit 3. Contact fingers 5a, 6a are contacted on a machining surface of the workpiece 1 so as to sense a machining size of the workpiece feeding mechanism for varying a feeding speed of the workpiece 1, the sensing signal A, B from the first and second measurement sensors 5, 6 are sent to the calculation circuit 7 in the control unit 3. In this arrangement, it is possible to improve an accuracy of size of the workpiece, prevent an excessive margin of it and provide a small-sized and low weight plainer device.