Sleepiness causes human errors and traffic accidents. It accounts for about 20% of all traffic accident factors. Attempts have been made to detect sleepiness during driving by using electroencephalograms (EEG) and reaction time to auditory and visual stimuli. It is, however, not realistic to apply these methods during driving. In the present study, we investigated the effect of sleepiness on the muscle force generation pattern in foot that steps on a device simulating a gas pedal of an automobile as to explore for the possibility of sleepiness detection. During the 45min experiment, 13 subjects was instructed to continuously demonstrate a net 20N muscle force on the device simulating the automobile's gas pedal. We also investigated subjective sleepiness by Stanford Sleepiness Scale. The coefficient of variation (CV) in force every minute was counted for each subjective sleepiness value. As a result, the ratio of CV less than 0.2 was high when the subjective sleepiness was not felt. In turn, the ratio of CV of 0.2 or greater, or negative value became high when strong sleepiness is felt. It was suggested that sleepiness could be detected by using CV of foot muscle force against a gas pedal.