To easily work a diamond pellet for grinding glass lens of spherical surface accuracy of a medium finish degree by even an unskillful person by changing a feed operation of a work spindle by a feed mechanism according to a detected current value of a work spindle load current detector by using a control circuit.

By holding a diamond pellet 10A (10B) in place of a glass lens on a work spindle 31 by using a spherical surface generating and working device of the glass lens, further detecting a working load current value during working and changing a feed operation of a work by a feed mechanism 33 according to the detected current value, a hard diamond pellet 10A (10B) can be worked. To detect working load current, rotary driving mechanisms 24, 32 are provided with motor current detecting devices 25, 35, respectively, and the devices 25, 35 are connected to a control circuit 36. The control circuit 36 controls feed speed of the work spindle 31 by the feed mechanism 33 according to an output of at least one of the motor current detecting devices 25, 35.