PURPOSE: To measure calorific value and Wobbe index of a fuel by keeping, in a mixing device to which a fuel of constant pressure and an ambient air of constant pressure are supplied, a fuel/air mixture of theoretical amount.

CONSTITUTION: The output fuel gas from an orifice 11 is, through a pressure regulator 15, supplied to a mixing device 19. An ambient air is sent from the first capillary tube 27 through the first stage air pressure regulator 47 and the second stage air pressure regulator 23, so that the total amount containing a variable air supplied through the second capillary 29 is a specified flow amount of fuel-air mixture, in theoretical amount, within the mixing device 19. The output from the mixing device 19 is, through a flow meter 31, supplied to a remaining oxygen sensor 35, for detection. Then, a controller 37 supplies a current signal which changes in response to the changes in the measured value of remaining oxygen to a converter 39, according to changes in input current from the controller 37, controls the air pressure from an pressure regulator 43, and supplies it as an air input to the mixing device 19 through the capillary tube 29.