A biosignal determining device includes: an instruction output circuit which outputs a first instruction, a second instruction, and a third instruction to a user, the first instruction is for asking the user to perform an inhaling or exhaling motion to a limit, a second instruction is for asking the user to stop the motion, and a third instruction is for asking the user to perform a motion reverse to the motion to a limit; a detection circuit which obtains a first cardiography representing a potential difference between two electrodes disposed on a chest of the user, the first cardiography measured between a time when the second instruction is outputted and a time when the third instruction is outputted, and detects a plurality of peaks included in the first cardiography; and a determination circuit which determines whether the user has performed the motion asked in the first instruction to the limit depending on whether potential values of the plurality of peaks are included in a predetermined range.