[en] Methods and results of absolute calibration of ionization chambers using nuclear emulsions are described. The absolute calibration consists in determination of the coefficient, which relates the chamber signal with a number (or flux density) of particles passing through it. The sensitivity of the reference chamber is measured with 3% accuracy and is equal to (2.22+-0.07)x10₆ muons/cm₂xV. It is shown that the dependence of the calibration coefficient on the depth and radial chamber position in a filter has not been revealed within the errors of every definite measurement (+-4%). The results of several calibration exposures during three years (1979-1981) are described by a straight line. The experiments carried out have just shown independently on proton beam intensity monitor a direct proportionality of ionizaiton chamber signals to muon flux density and the absence of any direct component, which might cause a systematic error during muon flux measurement