The present invention provides an optical apparatus comprising an optical gain correction filter having a multilayer film structure formed by stacking a plurality of thin films with different diffractive indexes on a light transmitting board, wherein when the light with the wavelength .lamda. enters at the incident angle .theta., the transmissivity is assumed to be T1 (.lamda., .theta.) (0.ltoreq.T1 (.lamda., .theta.).ltoreq.1), and the thickness and stacking state of each thin film of the optical gain correction filter are adjusted to increase the transmissivity T1 (.lamda..sub.0, .theta.) when the incident angle .theta. increases close to the predetermined maximum incident angle .theta.max with respect to the incident light with the wavelength .lamda..sub.0. The optical apparatus is applied to a bar code reader.
