Experimental measurements of the adiabatic burning velocity in methane-oxygen-nitrogen mixtures are presented. Non-stretched flames were stabilized on a perforated plate burner at I atm. The oxygen content in the artificial air was varied from 16 percent to 21 percent. The Heat Flux method was used to determine burning velocities under conditions when the net heat loss of the flame is zero. Major attention in this work has been paid to the identification of possible uncertainties and errors of the measurements. The overall error of the burning velocities is estimated to be smaller than 0.8 cm/s. Experimental results are in very good agreement with recent literature data for methane-air mixtures. They also agree well with detailed chemical model predictions.