An alertness device includes a heart rate sensor configured to obtain an electrocardiographic signal of a person, a calculation unit configured to calculate the electrocardiographic signal obtained from the heart rate sensor, a waveform generation unit configured to generate an RRI waveform from an electrocardiographic waveform of the electrocardiographic signal, and a determination unit configured to determine the alertness of the person based on the electrocardiographic signal. The calculation unit replicates a same number of RRI waveforms as that of anchors, each anchor being a point where a certain RRI is shorter than an adjacent preceding RRI, moves the time axes of the replicated RRI waveforms such that the anchors of the replicated RRI waveforms are in phase with each other, and calculates a PRSA signal defined as an RRI average for each time axis. The determination unit determines the alertness based on the RRIs and the PRSA signals.