We have developed a compact spectral sensor by using an interference filter. It is smaller and more stable for vibration and shock than one using gratings. This sensor is basically composed of the interference filter and silicon photodiode arrays. The composition of the filter is three layers of thin films, Ag/MgF₂/Ag, in which MgF₂ film is changed to controlled transmissible wave length continuously. We discussed in this paper mainly the construction of the interference filter and its optical characteristics.