We analyzed inadvertent human errors during 3-day trial examinations for the National Examination for Physicians. Sixth-year medical students sat for 2 different examinations consisting of 500 multiple-choice questions and chose either 1 or 2 correct answers. After the first examination, the students verified their errors and were provided with educational guidance to prevent inadvertent errors.1) More than half of the students made inadvertent errors during the examination.2)The errors occurred when the students solved questions or marked the answer sheets.3) Most of errors were either the selection of the wrong number of answer options (i.e., a 2-choice selection was required, but only 1 choice was selected) or the selection of choices that differed from the intended choices when the answer sheets were marked.4) After the students were taught how to avoid errors, the mean number of errors per examination per student decreased significantly from 2.1 to 1.0.5) To our knowledge, this is the first report to show the educational effectiveness of a method to decrease the rate of inadvertent errors during examinations.