To hold a substrate having parts to be machined on the opposite sides.

A handle part 2 is fixed to a circular chuck part 1 having a diameter substantially equal to that of a semiconductor wafer being chucked. An annular chuck surface 3 is provided on the outer circumferential part of the chuck part 1 to project from other parts thereof. A plurality of suction holes 4 are made in the chuck surface 3 and vacuum passages 5 are made in the chuck part 1 and the handle part 2. The vacuum passage 5 has one end connected with the suction hole 4 and the other end connected with one port of a solenoid valve 6 which can be operated independently. The solenoid valve 6 has the other port connected with a vacuum chamber 7 which is connected with a vacuum pump 8.