A derailment prevention guard includes a guard member (3) installed within a gauge and a support member (6) fixed to a sleeper (4) or a concrete slab track. The guard member (3) engages a hold member (8) moveable around an axis (7) supported by the support member (6) as turning center between a main rail (1) and the inside of the gauge on the sleeper. The support member (6) engages the hold member (8) by bolt (12) through turning the hold member (8) toward the main rail (1) around the central axis (7) as turning center on the sleeper (4), and the guard member (3) can be shunted inward by turning the hold member (8) toward the inside of the gauge around the central axis (7) as turning center on the sleeper (4) after loosening the bolt (12).