An alertness device with a high accuracy for determining alertness, a seat equipped with the alertness device, and a method for determining alertness are provided. The alertness device includes: a respiration sensor obtaining respiratory data of a person; a calculation unit calculating the respiratory data, a waveform generation unit generating an RI waveform which is transition in a predetermined time period of a respiratory interval (RI) which is an interval for one respiration; and a determination unit determining a state of alertness of the person on the basis of the respiratory data. 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 ², the determination unit determines on the basis of values calculated by the calculation unit that the person is in a state of low alertness.