A hydrogen gas generator comprises: an electrolyzer (2) configured to include a housing (20), a first chamber (21), a second chamber (22), a membrane (25), and a pair of electrode plates (23, 24); a tank (6) configured to store water to be electrolyzed (W); an electric power source (3) configured to apply a DC voltage to the pair of electrode plates; a diluter (4) configured to introduce a diluent gas into the first chamber or the second chamber in which the electrode plate to be a cathode is provided, the diluent gas diluting hydrogen gas generated; an electric quantity detector (51) configured to detect an electric quantity given to the electrode plate to be the cathode; a flow rate detector (52) configured to detect a flow rate of the diluent gas from the diluter; a calculator (5) configured to calculate a concentration of the diluted hydrogen gas on the basis of the electric quantity detected by the electric quantity detector and the flow rate detected by the flow rate detector; and an indicator (54) configured to present the concentration of hydrogen gas calculated by the calculator.