The base plate is transmissive to terahertz waves, and a sample is disposed at the base plate. In the conductive periodic structure, plural transmission portions that transmit terahertz waves are arrayed with a predetermined period. The conductive periodic structure is disposed apart from a position at which the sample is disposed. The waveguide includes a total reflection surface provided at a boundary face with the conductive periodic structure. The total reflection surface totally reflects incident terahertz waves, and the waveguide guides incident terahertz waves toward the total reflection surface. The magnitudes of one or more of a distance between the position at which the sample is disposed and the conductive periodic structure, a property of the base plate, and the predetermined period are set such that a dip showing a characteristic absorption is formed in a predetermined frequency region of a spectrum of terahertz waves.