Nuclear emulsion is a three-dimensional tracking detector with a sub-micron spatial resolution. Recently, the applications of nuclear emulsion have been expanding into imaging fields because of the improvement in scanning speed. Emulsion read-out systems exhibit an optical system- atic error in the angular measurement caused by the difference in refractive index. This error is conservatively estimated as 0.015 rad at an absolute track angle of 45°, which is a non-negligible value for the gamma-ray telescope experiment. We have evaluated this error experimentally using a test proton beam of 400 GeV at SPS/CERN. From the experimental results, the error decreased to 0.001 rad at an absolute track angle of 45°, which we will report on in this paper.