Described herein is a technique capable of uniformly processing substrates. According to the technique described herein, there is provided a substrate processing apparatus including: a process chamber where a substrate is processed; a gas supply configured to supply a gas into the process chamber; a plasma generator configured to plasma-excite the gas supplied into the process chamber, the plasma generator including an electrode electrically connected to a high frequency power source; an impedance meter configured to measure an impedance of the plasma generator; a determiner configured to determine an amount of active species generated by the plasma generator based on the impedance measured by the impedance meter; and a controller configured to control the high frequency power source based on the amount of active species determined by the determiner.