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The least-squares (LS) method is one of the most familiar and useful system identification method. However, there are many problems to be solved when one applies the LS method to the actual system. For example, it is difficult to identify lower frequency region accurately, because the cost function in the conventional LS method weights higher frequency region. This paper proposes a new identification method which uses multiple decimation operation, so we call it MD (MultiDecimation) method. The decimation, consists of the low-pass filtering and the resampling whose sampling rate is decreased, is widely used in the area of the signal processing. The advantages of the MD method are (a) By deviding the frequency region of the system, the characteristics in lower frequency region can be identified accurately as well as that in higher frequency region, (b) Data scaling can be carried out individually in each devided frequency region, and (c) By focusing on the narrow band, vibrational characteristics which are difficult to identify by the LS method can be identified accurately.In industrial robots, there are many vibrational modes which are caused by speed-reduction mechanisms, in the wide frequency range. It is necessary to identify these vibrational modes accurately so as to design stable servo controllers.So, we apply the MD method to the identification of robot arm, and we show the efficiency of the MD method through the identification experiments.