Abstract: There is provided is a transmitted light control device capable of controlling a peak wavelength and a peak intensity of transmitted light while keeping a sharp wavelength width. A transmitted light control device 10 includes: a grating substrate 1; a metal thin film 2; a conducting polymer layer 3 made by depositing a conducting polymer on the metal thin film 2; a cell 4 filled with a liquid medium 5 composed of an electrolyte or a buffer solution and configured such that a part of the liquid medium 5 is in contact with the conducting polymer layer 3; and a metal thin film potential control means 6 having a working electrode W connected to the metal thin film 2 and having a counter electrode C and a reference electrode R each connected to the liquid medium 5. The substrate 1 and at least a part of the cell 4 are made of a light transmitting material.