PURPOSE: To detect the moisture in a flux consisting essentially of a solvent except water and to detect the change in the compsn. thereof by measuring the change in the electrical characteristic of the flux at the set temp. and specific gravity of said flux.

CONSTITUTION: The easy detection of the moisture or extraneous material intruded into the flux is made possible by using a measuring instrument such as a conductivity meter, without requiring such a laborious operation as to take out a sample and to make the chemical analysis thereof to measure the change in the electrical characteristic of the flux and to detect the change in the compsn. of the flux. The measurement of the mixing amt. thereof as well as the continuous measurement are made possible as well. The instrument consists of a temp. detector 6, a specific gravity detector 5 and an electrical characteristic detector 7 connected to a control device 8 which are just required to be kept inserted into a flux tank 1. No intricate constitution is required for the instrument.