The invention relates to a detection method and an apparatus for detecting the state of a substance no matter having an inherent vibration spectrum of the band of a THz wave or not. The detection apparatus includes a sample holding section for holding the substance 8, an irradiation means 1 and 6, a detection means 1 and 7, a calculation means, and an evaluation means. The irradiation means 1 and6 irradiates the substance held in the sample holding section with a THz wave. The detection unit detects the THz wave passed through or reflected from the substance. The calculation unit determines the frequency dependence of a property of the substance 8 with respect to the irradiated THz wave and then calculates the slope of a straight line or the slope of a straight line obtained by straight-line approximation of the frequency dependence of the property of the substance. The evaluation unit evaluates the state change of the substance by comparing the previously-obtained slope of the straight line of the frequency dependence of the property of the substance in a standard state with the slope of the straight line of the substance calculated.