An alertness device comprising: a respiration sensor for measuring signals associated with respirations of a person; a waveform generation unit for generating respiratory data based on the signals obtained from the respiration sensor and for generating an RI waveform based on the respiratory data, wherein the RI waveform being a curve of a respiratory interval depending on the time, wherein the respiratory interval is an interval for one respiration; a calculation unit for calculating in a predetermined time period an average value of the respiratory interval and a Respiration root Mean Square Successive Difference (RrMSSDn), wherein the Respiration root Mean Square Successive Difference (RrMSSDn) is defined by the following formula: RrMSSDn = RI t − RI t − 1 2 + ⋅ ⋅ ⋅ ⋅ + RI t + n − 1 − RI t + n − 2 2, where the t-th respiratory interval in the RI waveform is determined as RI (t), and the n-th respiratory interval after the t-th respiratory interval in the RI waveform is defined as RI (t+n); and a determination unit for determining a state of alertness of the person that is configured to determine on the basis of values calculated by the calculation unit that the person is in a state of low alertness, if it is the case that an average value of the respiratory interval in a subsequent predetermined time period is greater than an average value of the respiratory interval in a predetermined time period directly previous to the subsequent predetermined time period, and if it is the case that the Respiration root Mean Square Successive Difference (RrMSSDn) of the subsequent predetermined time period is greater than a value which is obtained by multiplying the Respiration root Mean Square Successive Difference (RrMSSDn) of the predetermined time period directly previous to the subsequent predetermined time period by a β value. a respiration sensor for measuring signals associated with respirations of a person; a waveform generation unit for generating respiratory data based on the signals obtained from the respiration sensor and for generating an RI waveform based on the respiratory data, wherein the RI waveform being a curve of a respiratory interval depending on the time, wherein the respiratory interval is an interval for one respiration; a calculation unit for calculating in a predetermined time period an average value of the respiratory interval and a Respiration root Mean Square Successive Difference (RrMSSDn), wherein the Respiration root Mean Square Successive Difference (RrMSSDn) is defined by the following formula: RrMSSDn = RI t − RI t − 1 2 + ⋅ ⋅ ⋅ ⋅ + RI t + n − 1 − RI t + n − 2 2, where the t-th respiratory interval in the RI waveform is determined as RI (t), and the n-th respiratory interval after the t-th respiratory interval in the RI waveform is defined as RI (t+n); and a determination unit for determining a state of alertness of the person that is configured to determine on the basis of values calculated by the calculation unit that the person is in a state of low alertness, if it is the case that an average value of the respiratory interval in a subsequent predetermined time period is greater than an average value of the respiratory interval in a predetermined time period directly previous to the subsequent predetermined time period, and if it is the case that the Respiration root Mean Square Successive Difference (RrMSSDn) of the subsequent predetermined time period is greater than a value which is obtained by multiplying the Respiration root Mean Square Successive Difference (RrMSSDn) of the predetermined time period directly previous to the subsequent predetermined time period by a β value. where the t-th respiratory interval in the RI waveform is determined as RI (t), and the n-th respiratory interval after the t-th respiratory interval in the RI waveform is defined as RI (t+n); and a determination unit for determining a state of alertness of the person that is configured to determine on the basis of values calculated by the calculation unit that the person is in a state of low alertness,