To establish a method capable of continuously and comparatively evaluating the magnitude of an oxide film removal function (activity) of solder and a base material being the most significant role of a flux without being affected by a sample nor a measurer although the flux is used to improve the bondability of the solder in the case of performing solder joint.

The magnitude of flux activity can be continuously and comparatively evaluated without being affected by a sample nor a measurer by defining a difference (ΔT2-ΔT1) between a temperature difference or an absorption heating value ΔT1 of a flux at each temperature T which is obtained by heating the flux up to a soldering temperature and simultaneously measuring the temperature difference or the absorption heating value and ΔT2 obtained by heating the flux and solder chief material or a mixture of oxide power of a base material up to the soldering temperature and simultaneously measuring a temperature difference, a DTA value or a DSC value as an active parameter of the flux at each temperature, and making the integrated value thereof an evaluation index of activity.