A melody discrimination test was invented, mainly to survey train driver's mental workload, and has been applied to laboratory work and experimental train operations. Under the dual task condition, one of two melodies was presented to the subject every twenty seconds on average to judge whether it is the same as the preceding one as the secondary task. In this test, it decides the subject's performance whether he/she has memorized the presented melody. The results show that the test may be useful for estimation of mental workload of the primary task, when the data is analyzed with adequate grouping.