PURPOSE: To enable calculation of enthalpy with high accuracy by constituting a device in such a manner that the data of temperature and of relative humidity are inputted in a microcomputer and thereby the value of enthalpy is calculated so as to be indicated or outputted outside.

CONSTITUTION: The data of temperature 6 and the data of relative humidity 7 are inputted in a microcomputer 8, and the saturation vapor pressure hs is calculated by the following formula. An approximation formula: hs=At2+Bt+C, where A= 0.035, B=-0.172, C=6.514, and t is the temperature. Therefrom, the absolute humidity x is defined as; x=0.622×(h2ψ/100)÷(760-Hsψ/100), where ψ is the relative humidity. Next, the enthalpy i is determined as; i=0.24t+(597.5+0.44t)x. The determined enthalpy i is indicated in an indicating part 12, being outputted outside 13 as occasion calls. In addition, the data of absolute humidity 11, the data of temperature 6 and the data of relative humidity 7 are also outputted as occasion demands. Thereby, the calculation of enthalpy with high accuracy can be performed.