To provide a braking device of wheels which can prevent the runaway of vehicles with simply constructions like a wheel chair or baby car.

This device is constituted to fix a rotatable drum 2 in one body with a wheel center, and fix many numbers of rotating magnet 5 being a permanent magnets in the inner peripheral surface of the rotating drum along the peripheral direction and arrange the fixed disc 3 in the inner peripheral side of the rotating drum 2, and fit a braking device 4 to the fixed disc 3, and constitute the braking disc 4 to have an oscillating piece 6 and have a fixed magnet 7 being a permanent magnet which closely oscillate the rotating magnet 5 of the rotating drum 7, and the oscillating piece violently by a magnetic power which is caused when a great number of rotating magnet 5 continuously contact the fixed magnet 7 or leave from it according to the rotation of a wheel I in more than constant speed and move reciprocally a small piston 8 in a small cylinder 9, send a compressed air A generated by the movement into a large cylinder 11 and move a large piston 10 forwards, and brake or stop the wheel I by using the forwarding force.