To enhance the performance and reliability of sonic type instrumentation for the temperature/flow measurement of a gas, based on propagation time detection, by providing a method of identifying misdetection state in a propagation process, caused by superposition of disturbance noises and distortion of transmitted waveform, produced in the propagation process by conditions on physical properties inside a combustion furnace or inside a duct, keeping followability on the temperature/flow states.

This sonic type gas temperature/flow measuring method is provided with a data recording means which stores detected propagation times for a predetermined number of past data; a means for computing differential signal between for each of measured propagation time and the time one measurement period preceding it (last measurement time), with respect to a signal string from a past propagation time up to the present propagation time measured value; a means which performs a wavelet transformation for two kinds of data strings, a data string which does not contain the value measured this time and a data string containing the value measured this time, for evaluation with respect to the computed differential signals; and an error detecting means which determines normality/abnormality of the measured value for this time, to be evaluated by intensity comparison or pattern recognition of wavelet transformation performed for the two kinds of data strings.