The invention relates to an infrared modulator for spectrometer. It comprises a light source (1), a beam splitter (2) for splitting the light from the source into two beams, a first plane mirror (4) for directing the light of the first beam, a second plane mirror (5) for directing the light of the second beam, a first cube corner mirror (7) for turning the light back to the first plane mirror (4), and a second cube corner mirror (6) for turning the light back to the second plane mirror (5). The first and second cube corner mirrors (6, 7) are arranged on a common optic axis to reflect into opposite directions and movable back and forth in the direction of said optic axis. According to the invention, the beam splitter (3) and the first and second plane mirrors (4, 5) are supported by the same support structure formed by one uniform material piece.