To curb the manufacturing cost of a rail overturning prevention plate capable of exerting an accurate rail overturning prevention effect.

This rail overturning prevention plate (12) of a railroad rail overturning prevention device (1) comprises a rail groove (126) in which a rail (6) is to be arranged, a first inverted L-shaped arm (134) which is provided at the inside edge of the rail groove (126), and a rail reaction bearing seat (136). A leaf spring (14) fixed to the rail overturning prevention plate (12) is engaged with an outside lower end flange (6b) of the rail (6). The first arm (134) has a locking portion (134b) which is elongated to a base end of an inside lower end flange (6a) of the rail (6). A contour of an outer shape of the rail overturning prevention plate (12) is formed of cavities of male and female molds (15) and (16) of a molding tool, and the female mold (16) is provided with a protrusion (16a) for molding an inner surface of the first arm (134).