PURPOSE: To simplify a program, to effectively utilize a memory space and to process at a high speed by storing previously heat quantity data in response to the combination to be expected of temperature data and flowrate data in an external memory circuit, an constructing an address signal to read out the data by a combination of suitable assignment of digital conversion data of two input data.

CONSTITUTION: When the digital conversion values of temperature data and flowrate data are represented by variables x, y, the corresponding heat quantity data F can be directly read out as the correspondence between the address signal and the address content in an external memory circuit 7. For example, the heat quantity data having the prescribed calculation relation with two input data from the corresponding addres in the memory circuit 7 is directly read out by the digital data of a digitial converter 6 with respect to the flowrate Wq and feed water temperature Vc of a pulse period measuring circuit 4 as a flowrate measuring circuit, an outputted to a microcomputer 1 as a main calculator. The microcomputer 1 calculates gas quantity data Fg based on the read out and outputs it to a gas quantity controller 8.