To continuously measure the concentration of a dissolved gas without relying on recording ink.

A pressure reducing means 16 is connected to an ink tank 12 in which ink 11 is stored and a hermetically closed container 20 filled with pure water 19 for dissolving exhaustion is connected to the vacuum means. The dissolved gas of the ink is removed using the pressure reducing means to be dissolved in the pure water and the concentration of the dissolved gas in the pure water is measured to indirectly calculate the concentration of the dissolved gas of the ink from the whole conditions. Since the measurement of the concentration of the dissolved gas is performed through the pure water, a stable measurement over a long period of time becomes available even in the measurement of such a concentration of the dissolved gas of the ink as to destroy a probe.