A sleepiness assessment value P1 is set to be an average heart rate HR (S130). A sleepiness assessment value P2 is set to be a standard deviation σ of a respiration interval RespI for one minute (S150). A sleepiness assessment value P3 is set to be a value that is obtained by dividing, by one minute, an integrated value of a square of an average heartbeat interval RRIavg in a case where a variation in which a heartbeat interval RRI increases occurs (in a case where an RRI count value Xi is a value of one) (S190). A sleepiness assessment value P4 is set to a value that is obtained by averaging a variation ratio in a case where a variation in which the respiration interval RespI increases occurs (in a case where an RRI count value Yi is a value of one) (S210). A sleepiness assessment value P is set to be a value obtained by summing values that are obtained by multiplying the individual sleepiness assessment values P1 to P4 by weight coefficients a to d, each of which is set for a corresponding one of the sleepiness assessment values P1 to P4 (S220). When the sleepiness assessment value P is smaller than a value of zero, it is determined that a person is highly alert (S240), and, when the sleepiness assessment value P is equal to or larger than the value of zero, it is determined that the person is less alert (S250).