The acceleration sensors are generally attached to the structure to measure the response in FRF (frequency response function) measurement. The mass of the sensor often have some effect on dynamic characteristics of the structure. This paper proposes a new method for compensation of sensor's mass effect in FRF measurements. In the method, inertial force of the sensor's mass is regarded as external force acting on the structure; FRF is corrected so as to remove the external force effect. The compensated FRFs of an accuracy are evaluated by a Reliability Factor which it is installed in this paper. The compensation method for the use of multiple sensors is also presented. The method is validated by applying it to a brass plate structure. The usability of the method is demonstrated via measuring FRFs of a car body.