A method of manufacturing a semiconductor device includes forming a film along a surface of a semiconductor substrate in a first surface area state having a first surface area by supplying a reaction gas at a first flow rate. The method further includes detecting a transition from the first surface area state to a second surface area state having a second surface area different from the first surface area. The method still further includes forming the film by changing the flow rate of the reaction gas from the first flow rate to a second flow rate different from the first flow rate after detecting the transition from the first surface area state to the second surface area state.