This paper proposes a simple passivity-based disturbance rejection scheme for force-controllable biped humanoids. The disturbance rejection by force control is useful not only for self-balance, but also for stable and safety physical interaction between human and humanoid robots. The core technique is passivity-based contact force control with gravity-compensation. This makes it easy to control the contact forces in a satisfactory dynamic range without canceling all non-linear terms. The disturbance rejection is located at the higher layer above the contact force controller. It is composed of three sub-controllers; 1) a balancing controller; 2) a stepping controller; and 3) the trigger. Numerical simulations and experiments evaluate the effectiveness of the proposed controller. Although the method is incomplete in the sense that the self-collision between the limbs is ignored, a preliminary experimental result on a real humanoid platform demonstrates that the proposed method can actually make the robot recover the balance under large unknown external perturbations.