A method of measuring the thermal conductivity of materials during transient heating or cooling is proposed based on the continuous recording of the temperature difference between sample surfaces when known heat flux is supplied to one of the sample surfaces by a heater. The analysis of the solution of the heat conduction equation for a measuring system consisting of two platelike samples separated by a heater shows that the transient heating or cooling significantly influences the temperature distribution within the samples. It follows from the theoretical consideration that this effect as well as the effects resulting from heat generation or absorption in the samples during phase transitions can be eliminated by subtracting the results of two measurements conducted with different powers supplied by the heater. The condition for validity of this procedure is the small temperature difference between sample surfaces which can be achieved by the selection of appropriate temperature change rate or sample thickness.