In this paper, a ZMP (Zero-Moment Point) disturbance observer for walking stabilization of the biped robot is proposed. The disturbance to a ZMP is defined as a "ZMP disturbance". The ZMP disturbance is composed of a modeling error and external force. The ZMP disturbance observer divides the ZMP disturbance based on the frequency band, and applies a position compensation method and an acceleration compensation method. The ZMP disturbance observer is effective to the ZMP disturbance, and useful for the walking stabilization of the biped robot. Additionally a method of converting the compensation to a COG (Center of Gravity) trajectory into the compensation to a body trajectory is proposed. The validity of the proposed method was confirmed in the simulations and the experiments.