A calibration method for a thrust load measuring device of a hermetically sealed kneader, the hermetically sealed kneader including a pair of rotors that are disposed to be adjacent to each other with a predetermined space therebetween in such a manner that axes are parallel to each other and rotate in directions different from each other, in which a bearing (6, 7) that supports a load applied to each rotor in a radial direction is provided at both end sides of the pair of rotors, and a load applied to each rotor in a thrust direction is supported by the bearing (6, 7) on one end side among from the bearings (6, 7) at both the end sides, the method comprising: determining a relative displacement along an axial direction between an outer race fixing component that fixes an outer race of the bearing on the one end side or a casing to which the outer race fixing component is fitted, and an inner race fixing component that fixes an inner race of the bearing on the one end side or the rotor to which the inner race fixing component is fitted; using axial force measuring bolts that can measure a load applied in the axial direction as fastening bolts for fixing the bearing on the one end side when a thrust load applied to the rotor is calculated by multiplying the determined relative displacement by a conversion factor; and calibrating the conversion factor using axial forces measured by the axial force measuring bolts and the relative displacement when the axial forces are measured, characterized in that the conversion factor is calibrated using axial forces measured by the axial force measuring bolts when an axial force above an initial axial force is applied to all the axial force measuring bolts, further characterized in that a load less than or equal to a value in which a maximum thrust load among from thrust loads generated on the rotor during kneading is divided by the number of the axial force measuring bolts provided to the casing is set to be the initial axial force. determining a relative displacement along an axial direction between an outer race fixing component that fixes an outer race of the bearing on the one end side or a casing to which the outer race fixing component is fitted, and an inner race fixing component that fixes an inner race of the bearing on the one end side or the rotor to which the inner race fixing component is fitted; using axial force measuring bolts that can measure a load applied in the axial direction as fastening bolts for fixing the bearing on the one end side when a thrust load applied to the rotor is calculated by multiplying the determined relative displacement by a conversion factor; and calibrating the conversion factor using axial forces measured by the axial force measuring bolts and the relative displacement when the axial forces are measured, characterized in that the conversion factor is calibrated using axial forces measured by the axial force measuring bolts when an axial force above an initial axial force is applied to all the axial force measuring bolts, further characterized in that a load less than or equal to a value in which a maximum thrust load among from thrust loads generated on the rotor during kneading is divided by the number of the axial force measuring bolts provided to the casing is set to be the initial axial force.