To provide an automatic braking device which can be easily mounted on a wheelchair or the like and which operates mechanically without using electricity.

The automatic braking device comprises a speed sensing mechanism 10 which is attached to a wheel 3 of the wheelchair 1 or the like and which operates when the speed of the wheelchair 1 or the like exceeds a certain speed, and a braking mechanism 20 which is attached to a wheelchair main body 2 and which brakes the wheel 3 in cooperation with the speed sensing mechanism 10. The speed sensing mechanism 10 comprises a cylinder 11, a piston 14 reciprocating within the cylinder, a flow rate regulator 17 arranged in a ventilation passage 15 communicating with the air within a space defined by the piston within the cylinder, and a valve 12 to be opened/closed for introducing or discharging the air. The braking mechanism 20 senses the height of the piston 14 corresponding to the speed of the wheelchair 1 sensed by the speed sensing mechanism 10, and brings a brake block 24 into contact with the wheel 3a mechanically to thereby control the speed of the wheelchair 1 or the like.