An alertness device includes: a respiration sensor that obtains respiratory data of a person a calculation unit a waveform generation unit that generates a respiratory interval (RI) waveform which is a transition in a predetermined time period of an RI which is an interval for one respiration and a determination unit. The calculation unit calculates an average value of the RI and RrMSSDn in a predetermined time period. In a case where an average value of the subsequent RI is greater than an average value of the RI directly previous to the subsequent RI and in a case where the RrMSSDn of the subsequent RI is greater than a value which is obtained by multiplying the RrMSSDn of the previous RI by a constant β, the determination unit determines on the basis of values calculated by the calculation unit that the person is in a state of low alertness.
