The work load given to workers by a monotonous task was investigated. Their load was obtained by measuring the flicker value and observing its fluctuation pattern. Each flicker fluctuation was classified into one of the three patterns. It was found that the mental load of the subject carrying out the task by himself tends to become lighter according as the speed increases, while the mental load of the subject doing the task with his companion does not show this tendency. On the other hand, the scores of the disposition tests were analyzed with regard to the flicker fluctuation pattern by using the discriminant function.