To provide a cutter facilitating the cutting of a lightweight channel steel by a small force.

This cutter for the lightweight channel steel is provided with a receiver tooth 2 having a pair of channel cutting blades 3 opposed to each other at a prescribed interval apart from each other and a pushing tooth 5 having an angle cutting blade 6 fitted in the channel cutting blades in such a state as freely appearing therefrom/disappearing therein. The receiver tooth 2 and the pushing tooth 5 are rotationally connected together in respective one end part sides, the crown part 6a of the angle cutting blade 6 of the pushing tooth 5 is positioned in the central part of the groove width of the respective channel cutting blades 3, a first lever 10 is integrally connected to the receiver tooth 2, a second lever 11 is rotationally connected to the base end of the first lever 10, and a feed mechanism 13 rotationally moves the pushing tooth 5 in the direction of the receiver tooth 2 when the second lever 11 moves rotationally in the direction of the first lever 10.