Abstract: A semiconductor device manufacturing method includes forming a thin film containing silicon, oxygen, carbon and a specified Group III or Group V element on a substrate by performing a cycle a predetermined number of times. The cycle includes: supplying a precursor gas containing silicon, carbon and a halogen element and having an Si—C bonding and a first catalytic gas to the substrate; supplying an oxidizing gas and a second catalytic gas to the substrate; and supplying a modifying gas containing the specified Group III or Group V element to the substrate.