To provide a lower arm part structure of a slide door, which restrains the slide door from intruding to the cabin inside during lateral collision.

The lower arm part structure of the slide door is provided with: a door beam 1 which is disposed at the upper side and extended in the longitudinal direction; a door reinforcement sash 5 where the end part of the door beam 1 and the lower arm 3 are joined and a closed cross section structure is formed in the vertical direction in cooperation with the front part of the door panel; and a separator member 4 which is provided between a joint part with the end part of the door beam 1 of the door reinforcement sash 5 and the joint part with the lower arm 3 and separates the inside of the cross section of the door reinforcement sash 5 in the vertical direction. Since the rotation deformation in the vicinity of a lower arm member 3 in the lateral collision is thereby suppressed, and the door reinforcement sash 5 composing a door pillar is not recklessly deformed by ensuring the cross section without largely deforming it in the initial stage of the collision, the deformation mode is made suitable.