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The paper examined response of brain wave to mental task and acoustic stimuli in order to evaluate stress effects of mental work and acoustic environment on brain electrical activity. The amplitude of the alpha wave (8-13 Hz), which is the representative component of awake brain wave, was decreased by approximately 15~20 % during performing the mental task (mental arithmetic and memorization) alone and listening to acoustic stimuli (5 KHz tone as unpleasant sound and a classical music, Pachelbel's Canon as a pleasant sound) alone in comparing to the amplitude of the spontaneous alpha wave at rest. Such amplitude suppression corresponds to increase of temporal phase fluctuation of the alpha wave by 15~30%. In particular, the reduction rate of the amplitude of the alpha wave in the low-frequency range was up to 25 % under the combined stresses, namely, the mental arithmetic and unpleasant 5 KHz tone. On the other hand, the amplitude of the beta and gamma waves showed no significant change under most of stress conditions including combined stresses. The observation obtained indicates that the combination of acoustic and mental stresses considerably affects the stabilization of the alpha wave in the low frequency range which may be a crucial key to measure the mental stress using the brain waves.