We investigated the image of light reflected by a pyramidal mirror with a vertex angle of 70.5 degrees and a length of one side of 1.5 mm. The pyramidal mirror was produced by using photolithography, bulk etching of silicon, and electroplating. As a result, three different types of reflection paths were observed, and the changes in the polarization of the reflected light were measured for each of the three types. In addition, the polarization change was theoretically calculated by using the Fresnel equation with the reflection angle for each of the three types and the complex refractive index of the mirror. The validity of the measured values was verified through comparison with the theoretical values, and an analytical model for the change in the polarization caused by reflection from a pyramidal mirror with a vertex angle of 70.5 degrees was developed.