Biological conversion of methane gas has been attracting considerable recent interest. However, methanotropic bioreactor is limited by low solubility of methane gas in aqueous solution. Although a large mass transfer coefficient of methane in water could possibly overcome this limitation, no dissolved methane probe in aqueous environment is commercially available. We have developed a reactor enabling the measurement of aqueous phase methane concentration and mass transfer coefficient (kLa). The feasibility of the new reactor was demonstrated by measuring kLa values as a function of spinning rate of impeller and flow rate of methane gas. Especially, at spinning rate of 300 rpm and flow rate of 3.0 L/min, a large kLa value of 102.9 h.1 was obtained.