An electric grinding machine includes a spindle to be driven with a motor; a grinding disc attached to the spindle; a cover covering the grinding disc; a window disposed on the cover to expose an outer circumference of the grinding disc; a hinge mechanism joining the window to the cover so that the window freely moves between an opened position and a closed position, said hinge mechanism having a rotational axis disposed on the cover; a gate protruding from the window and having a cylindrical portion concentrical to the rotational axes; a hollowed portion formed in the gate, said hollowed portion forming a passage opening between the cover and the cylindrical portion when the window is closed; a slider disposed on the cover, said slider passes through the passage opening to hold the window in the closed position; and a piston spring urging the slider toward the window.