Scattered radiation was measured following the application of a ceiling pendent-type lead acrylic protective board (protective board) for use in IVR, and the dose-reduction rate was obtained. As a result of the interaction between the protective board and the scattered radiation, the dose reduction rate in the space between the protective board and floor surface became negative, resulting in an increase in scattered radiation. It was demonstrated that the increase in scattered radiation could be prevented by installing an added filter. This report presents a method to evaluate the effect of a protective board on scattered radiation by converting its property to that of a material with a similar effective density, and explains the experimental results. This report also shows that a conventional method, which calculates scattered radiation on the basis of lead equivalent, carries the risk of underestimating the effect on scattered radiation. The necessity of training operators based on the above findings is also emphasized.