To provide a side trimming device capable of exactly performing side trimming of a steel plate without replacing an upper pressing ring (upper rubber ring) mounted on a side surface of an upper blade.

When the distance between the upper surface of the steel plate 1 and the lower end of an upper plate pressing roll 13 is R, the distance between the side surface of the upper rubber ring 12 and the side surface of the upper plate pressing roll 13 is L and the width of the upper plate pressing roll 13 is W in this side trimming device, a relation of 1 mm≤R≤2 mm, L≤50 mm, and W≥100 mm is satisfied.