A power-driven impact drill includes a spindle extending at a right angle to the axis of rotation of a drill bit, a rotational first hammer slidably mounted on the spindle for co-rotation therewith, and a reciprocal second hammer slidably disposed in front of the drill bit. When the rotation of the first hammer is arrested by the second hammer, the first hammer is caused by a cam mechanism to retract along the spindle in a direction away from the second hammer against the force of a first hammer spring. Upon disengagement with the second hammer, the first hammer is permitted by the cam mechanism to slide back into engagement with the second hammer under the force of the first hammer spring while it is rotated by the spindle. In this instance, the angular velocity of the first hammer received by the spindle is enhanced by the force stored in the compressed first hammer spring so that a large impact power is imposed to the rotating drill via the second hammer without substantial loss.