A wakeful-state data generating apparatus includes at least one processor and at least one storage device, the at least one storage device configured to; store a first correlation between value of variation in heartbeat interval and maximum spectral density, and store a second correlation between heartbeat rate and wakeful-state maximum frequency, the at least one processor configured to; calculate a value of variation in heartbeat interval from a heartbeat signal of a subject, estimate a wakeful-state maximum spectral density of the subject, on the basis of the first correlation and the calculated value of variation, calculate a heartbeat rate from the heartbeat signal, estimate a wakeful-state maximum frequency of the subject, on the basis of the second correlation and the calculated heartbeat rate, and generate wakeful-state data including the estimated wakeful-state maximum spectral density and the estimated wakeful-state maximum frequency.