Provided are a slag crusher, a gasifier, an integrated gasification combined cycle, and an assembly method of a slag crusher that can ensure the strength of a guide rod. The slag crusher includes: a porous member screen; a spreader that is reciprocated in a predetermined direction along a top surface of the screen and crushes the slag accumulated on the screen; and a guide rod having an axis line along the predetermined direction, is connected to the spreader, and restricts a moving direction of the spreader, the guide rod has a spreader-side member connected to the spreader and a shaft member connected to the spreader-side member, the spreader-side member and the shaft member are connected by butt welding in the axis line direction, and the spreader-side member and the shaft member have the same shape of cross sections orthogonal to the axis line direction at a butt welding position.