There is disclosed an apparatus comprising: a terahertz wave generation device which generates a terahertz wave; a terahertz wave irradiation device which irradiates an object to be inspected with the terahertz wave; and a scattering intensity detection device which cuts a rectilinear wave of the terahertz wave which has passed through the object to be inspected and which detects an intensity of a scattered wave, and a scattered material such as powder or foam contained in an envelope, a capsule, a container or the like is detected in a non-destructive manner without unsealing the envelope or the like.