Abstract: The invention relates to a portable, battery-driven device for measuring heat transfer coefficients (k-values). It uses an amplifier (1), which forms the quotient between the inner temperature minus the inner wall temperature on one hand and the inner temperature minus the outer temperature on the other hand. The temperature values of the first-mentioned substraction are obtained by thermal elements (7, 8), which are connected to the device. The difference according to the second one of the substractions is adjustable connected in a feed back circuit of the amplifier (1). The output quantity of the latter is supplied to an instrument (3) graduated in k-value. The thermal element for the measuring of the inner wall temperature can be provided at the tip of a probe (9), and may consequently be conveniently applied to different places of a wall to be investigated.