To provide a trailer falling-off prevention device capable of detecting danger of falling off a trailer when separating the trailer and of securely preventing the falling off.

At the time of work for separating the trailer 2, a connection detecting part 3 detects a non-connected state of the trailer 2 with respect to a tractor 1 and outputs a non-connected state signal S_NJ. A load detecting part 4 detects a load of the trailer 2 applied to a connection part and outputs a load signal S_L. When receiving the non-connected state signal S_NJ from the connection detecting part 3 and detecting that the load signal S_L received by the load detecting part 4 exceeds a predetermined threshold value, a controller 5 forcedly gives a control signal S_C to a brake part 6 to generate brake force. Alternatively, the controller 5 gives the control signal S_C to a brake control valve 63. The brake control valve 63 outputs atmospheric pressure P2 regardless of presence/absence of output of a hand brake part 62, and a brake force control part 63 receives the output atmospheric pressure P2 and then generates brake force.

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