Described herein is a technique capable of improving a quality of a film formed on a substrate. According to the technique described herein, there is provided a method of manufacturing a semiconductor device, including: (a) exhausting an inner atmosphere of a first gas supply pipe configured to supply a first gas generated by a vaporizer to a process chamber accommodating a substrate by a first gas discharge system connected to the first gas supply pipe; (b) exhausting an inner atmosphere of the vaporizer through a second gas discharge system provided at a vaporizer outlet pipe of the vaporizer by supplying an inert gas to the vaporizer via a vaporizer inlet pipe of the vaporizer; and (c) supplying the first gas generated by the vaporizer to the process chamber accommodating the substrate via the vaporizer outlet pipe, the first gas supply pipe and a first timing valve provided at the first gas supply pipe before (a) is performed or after (b) is performed.