In a bending method of bending a pipe material of metal in pipe working, at first particles are fed into a flow passage of the pipe material, the particles being removable in a liquid phase. The pipe material with the particles is bent. The particles in an at least partially liquid phase are removed from the pipe material, to clean the flow passage. The particles are meltable, or soluble in dissolving liquid. Specifically, in the removing step, the dissolving liquid is supplied into the flow passage, the particles are dissolved in the dissolving liquid, and compressed air is blown in the flow passage to eject the fluid obtained by dissolving the particles in the dissolving liquid. The particles are granulated sugar, and the dissolving liquid is warmed water.