PURPOSE: To obtain a horizontal low-pressure CVD device that can be cleaned without removing a reaction chamber from the device and its cleaning method.

CONSTITUTION: A deposit adhering to an inner wall of a quartz pipe 3 is removed by introducing a fluorine-containing compound gas, for example ClF3 gas, from a ClF3 gas supply source 20 into the quartz pipe 3 along with carrier gas. Introduction-to-discharge of ClF3 gas, etc., is carried out from the both directions of the quartz pipe 3 by a flip-flop method by valves 15a, 15b, 16a, 16b. Thus, particles generated from a reaction chamber, etc., can be reduced and VLSI having a high reliability can be obtained.