The thermal resistance of adiabatic material sometimes degenerates after a long time use. For old buildings, it is important to measure the thermal transmittance (U-value) of the walls. The infrared camera method was often used as nondestructive in situ measuring method enabling simple field test. The purpose of this paper is to estimate the feasibility of this method using numerical simulation. At first, in situ measuring of wall resistance value in a typical residential house was carried out to validate the results of numerical simulation. After the validation, the difference of thermal transmittance among the positions in the same wall was simulated. Moreover, this paper examined how much the different heater positions affect the result of thermal transmittance of wall. The simulation results confirmed that measurement accuracy can be sufficiently guaranteed. However, there is the risk of error in the thermal transmittance measurement, if an indoor heater is located too near the measurement wall in the room.