A drowsiness detection method that causes a computer to execute a process, the process includes: performing frequency analysis on heartbeat data obtained from a subject and determining a state of the subject by calculating entropy in a predetermined frequency bandwidth with a peak as a reference, where the peak is within a predetermined frequency range of frequency distribution obtained from the frequency analysis result, determining that the subject is in a relaxation state in a case where the calculated entropy is less than a predetermined threshold value, and determining that the subject is in a drowsiness state in a case where the entropy is equal to or greater than the threshold value.
