In order to improve measurement accuracy by accurately calibrating sensitivity of a dissolved oxygen meter without impairing easiness in calibration, the present invention includes a dissolved oxygen measuring device that includes a cell to be loaded with a sample solution, and a dissolved oxygen meter that is attached to the cell and measures an oxygen concentration in the sample solution; and a mode switching mechanism to switch between a measurement mode, in which a sensor surface of the dissolved oxygen meter is substantially in contact with the sample solution in the cell, and a calibration mode, in which the sensor surface is substantially not in contact with the sample solution in the cell, by feeding air into the cell through a feed port disposed on the cell, and by discharging part of the sample solution through a discharge port disposed on the cell.