Reliability is an important index to assess the validity of an optical instrument, which is directly related to the practicability of the equipment. Therefore, it is very important to study the reliability of the Mueller imaging polarimeter in pathological diagnosis for its clinical application. In this paper, the Mueller imaging polarimeter which is developed independently by our laboratory is used to diagnose 155 cases of skin tissue sections, including melanoma, pigmented nevus, and normal skin. The depolarization, retardance, and diattenuation can be obtained after the polarimetry. By analyzing receiver operating characteristic (ROC) curves, we find that the diagnostic reliability of depolarization, retardance, and diattenuation of normal and melanoma skin tissue sections is 100%, 100%, and 90%, respectively. The diagnostic reliability of depolarization, retardance, and diattenuation of normal and pigmented nevus skin tissue sections is 99%, 88%, and 75%, respectively. The diagnostic reliability of depolarization, retardance, and diattenuation of pigmented nevus and melanoma is 96%, 97%, and 78%, respectively. The reliability and validity of our homemade Mueller imaging polarimeter have been demonstrated in diagnosing skin sections and the instrument is expected to be a potential tool to assist doctors in pathological diagnosis.