As the first stage of the research into factors determining the optimum listening level of music performance, its methodology was examined. In experiment 1, a part of Symphony No. 6 in b-minor Op. 74, ``Pathetique,'' composed by Tchaikovsky, was used and the instantaneous impressions of loudness were judged using the method of continuous judgment by categories developed by Namba and Kuwano. A high correlation coefficient was found between LA[inf eq], 100 ms, and instantaneous impression of loudness sampled every 100 ms, which confirms the applicability of the method to the loudness of music performance. In experiment 2, musically significant portions whose durations were 5 to 20 s, were selected from the stimulus used in experiment 1 and rearranged in descending series and ascending series according to their sound levels with about 5-s silent intervals. The instantaneous impression of their loudness was judged as in experiment 1. It was found that each block in the ascending series was judged louder than the corresponding block in the descending series. It was suggested that the loudness of musical performance is significantly affected by the context.