A sleepiness level detection device detects a heartbeat signal when a time equal to or longer than T1 and equal to or shorter than T2 has elapsed since the start of driving. The heartbeat signal is subjected to FFT processing to obtain a spectrum signal. By use of a peak frequency of the spectrum signal, a driver conscious-state peak frequency is estimated. A consciousness level index band .alpha. is set with respect to the conscious-state peak frequency. A sleepiness level index band .beta. is set with respect to a frequency, which is calculated by multiplying the conscious-state peak frequency by a predetermined ratio (65 to 90%). A sleepiness level evaluation parameter Sp (=.beta.p/(.alpha.p+.beta.p)) for indicating the sleepiness level of the driver is calculated with the use of the strength .alpha.p and .beta.p of spectrum signals.