A method includes: acquiring, via a network, biogas information at multiple timings and time information corresponding to time at each of the multiple timings, wherein the biogas represents a concentration of furfural of a user acquired by a sensor that detects the furfural discharged from a skin surface of the user； obtaining reference information representing a lower limit of a normal range of furfural per unit period of time, using a memory storing the reference information representing the lower limit of the normal range； determining a stress time period during which a concentration of the furfural of the user is less than the lower limit of the normal range, based on the acquired biogas information； and outputting time period information indicating the determined stress time period to an information terminal of the user.
