PURPOSE: To surely and efficiently remove the substance strongly bonded to the inner surface of piping without generating clogging.

CONSTITUTION: A mixture wherein a granular material is mixed with a washing solution is received in a solid-liquid storage tank 23 and sucked in a solid-liquid mixture pressure feed device 1 to be fed to piping 21 under pressure and the mixture passed through the piping 21 is returned to the solid-liquid storage tank 23. By feeding the washing solution containing the granular material to the piping 21 under pressure, the granular material collides with the substance bonded to the inner surface of the piping 21 to remove said substance and even a strongly bonded substance is surely and efficiently removed without generating clogging.