To provide a braking device for automatically reducing a speed on a downward slope and preventing retreat of a simple vehicle on an upward slope.

This braking device 10 of wheels of the simple vehicle for controlling rotation of the wheels 58 respectively installed on a simple vehicle body via a fixing shaft 12, is provided with a gear mechanism 16 being arranged inside a hub 13 of the wheels 58 and using the rotation of the wheels 58 as input, a rotary plate 17 rotatably arranged on the fixing shaft 12, connected to the gear mechanism 16 and rotatable at a rotating speed different from the hub 13 and a speed reduction means 19 being movably arranged in the radial direction on the rotary plate 17, moving to the radial directional outside by centrifugal force by the rotation of the wheels 58 and reducing a speed of the wheels 58 by abutting to an inner peripheral surface 16a of the hub 13.