An analysis method for a substrate processing apparatus capable of accurately detecting a state in a housing chamber. Emission intensities of processing gas before being introduced into the chamber and processing gas having passed therethrough are measured before an inter-chamber part is replaced. If an emission intensity measured after the replacement coincides with that measured before the replacement, an emission intensity of the processing gas having passed through the chamber is measured, and a variation between the emission intensities of the processing gas having passed through the chamber measured before and after the replacement is calculated. After start of plasma processing on wafers, an emission intensity of the processing gas having passed through the chamber is measured and the variation is removed therefrom to calculate an emission intensity really representing a state in the chamber, thus detecting an end point of plasma processing therefrom.