surface 13 is capable of sliding and capable of transmitting a driving force the friction of the sliding surface 13 is larger than the friction of the bearing 9 when the rotational speed of the eccentric rotation shaft 4 about its axis increases considerably the sliding surface 13 functions as a brake when the rotational speed of the eccentric rotation shaft 4 about its axis decreases the sliding surface 13 functions to transmit a driving force a force that presses the clutch members 11 and 12 against each other can be adjusted by adjusting the elastic forces of the push springs 16 braking power and the ability to transmit a driving force can be increased by using springs having larger elastic forces or by compressing the springs more strongly braking power and the ability to transmit a driving force can be decreased by making adjustment in the opposite way the eccentric rotation shaft 4 is attached through the bearing 9 in such a way that a gap 15 is formed between the rotary disc 7 and the grinding disc side clutch member 12 so that the function of the sliding surface 13 described above may not be hindered as described above the clutch members 11 and 12 are partially engaged all the time and the sliding surface 13 is capable of sliding and capable of transmitting a driving force the grinding tool having such a combination of clutch.