This invention makes use of a electronically controlled heating device to form a region of constant high temperature on one side of the insulating material, and causes most of the heat transfer to concentrate in a longitudinal heat flux, which flows across the direction of the thickness of object-to-be-tested, to achieve the effect of one-dimensional heat transfer. When the temperatures on both sides of the insulation material being tested come to a stable state, the temperatures, the thickness of the material, and the heat flux, are measured and used to calculate the heat conductivity. The difference between the ideal two-dimensional heat transfer and the one-dimensional heat transfer can be corrected by the results of a theoretical model. The apparatus has been used to test a material with known heat conductivity. The result of the test conforms well with the expected value. Currently, the prototype which has been developed may be designed with a digital circuit which consists mainly of a single-chip microcomputer to perform PWM (Pulse Width Modulation) temperature control, temperature measurement, parameter input and coefficient calculation and displays functions.