An alertness device comprising: a heart rate sensor configured to measure an electrocardiographic signal of a person; a waveform generation unit for generating an electrocardiographic waveform based on the signals measured with the heart rate sensor and for generating based on the electrocardiographic waveform an RRI waveform being a curve of R-R intervals between R-waves depending on the time; a calculation unit for replicating the same number of RRI waveforms as that of anchors, each anchor being a point where a certain R-R interval between R-waves is shorter than a next preceding R-R interval between R-waves in the RRI waveform, for horizontally moving the time axes of the replicated RRI waveforms such that the anchors of the replicated RRI waveforms are in phase with each other on the horizontal axis, and for calculating a phase-rectified signal averaging signal (PRSA signal), which is an average of the R-R interval between R-waves of the replicated RRI waveforms for each index on the horizontal axis; and a determination unit configured to determine an alertness of the person, whether the person is in an alert state or an unalert state based on the R-R intervals between R-waves and the phase-rectified signal averaging signals (PRSA signals); wherein the waveform generation unit generates a PRSA signal waveform based on the phase-rectified signal averaging signals (PRSA signals); the calculation unit calculates in a predetermined time period in the RRI waveform an average value of the R-R intervals between R-waves and an average of an ACn, wherein the ACn is defined by the following formula: ACn = X 0 + X 1 − X − 1 − X − 2, where in the generated PRSA signal waveform, a PRSA signal at a certain index is defined by X, a PRSA signal for a heart rate at a next succeeding index of the certain index is defined by X, a PRSA signal for the heart rate at a next preceding index of the certain index is defined by X(-1), and a PRSA signal for the heart rate at a next preceding index of the index at which the X(-1) is obtained is defined by X(-2); and the determination unit determines that the person is in the unalert state if it is the case that the average value of the R-R intervals between R-waves in a subsequent predetermined time period less than an average value of the R-R intervals between R-waves in a predetermined time period directly previous to the subsequent predetermined time period, and if it is the case that the ACn average of the subsequent predetermined time period is less than a value which is obtained by multiplying the ACn average of the predetermined time period directly previous to the subsequent predetermined time period by an α value. a heart rate sensor configured to measure an electrocardiographic signal of a person; a waveform generation unit for generating an electrocardiographic waveform based on the signals measured with the heart rate sensor and for generating based on the electrocardiographic waveform an RRI waveform being a curve of R-R intervals between R-waves depending on the time; a calculation unit for replicating the same number of RRI waveforms as that of anchors, each anchor being a point where a certain R-R interval between R-waves is shorter than a next preceding R-R interval between R-waves in the RRI waveform, for horizontally moving the time axes of the replicated RRI waveforms such that the anchors of the replicated RRI waveforms are in phase with each other on the horizontal axis, and for calculating a phase-rectified signal averaging signal (PRSA signal), which is an average of the R-R interval between R-waves of the replicated RRI waveforms for each index on the horizontal axis; and a determination unit configured to determine an alertness of the person, whether the person is in an alert state or an unalert state based on the R-R intervals between R-waves and the phase-rectified signal averaging signals (PRSA signals); wherein the waveform generation unit generates a PRSA signal waveform based on the phase-rectified signal averaging signals (PRSA signals); the calculation unit calculates in a predetermined time period in the RRI waveform an average value of the R-R intervals between R-waves and an average of an ACn, wherein the ACn is defined by the following formula: ACn = X 0 + X 1 − X − 1 − X − 2, where in the generated PRSA signal waveform, a PRSA signal at a certain index is defined by X, a PRSA signal for a heart rate at a next succeeding index of the certain index is defined by X, a PRSA signal for the heart rate at a next preceding index of the certain index is defined by X(-1), and a PRSA signal for the heart rate at a next preceding index of the index at which the X(-1) is obtained is defined by X(-2); and the determination unit determines that the person is in the unalert state if it is the case that the average value of the R-R intervals between R-waves in a subsequent predetermined time period less than an average value of the R-R intervals between R-waves in a predetermined time period directly previous to the subsequent predetermined time period, and if it is the case that the ACn average of the subsequent predetermined time period is less than a value which is obtained by multiplying the ACn average of the predetermined time period directly previous to the subsequent predetermined time period by an α value.