A wearable assist robot apparatus, comprising: (a) an upper body trunk harness to be worn and retained on an upper body trunk of a wearer; (b) a lower body trunk harness to be worn and retained on a lower body trunk of the wearer; and (c) thigh harnesses to be worn and retained on thighs of the wearer, (d) drive sources disposed on the lower body trunk on both sides in a lateral direction of the wearer, the drive sources each including a drive shaft configured to rotate about an axial line in the lateral direction at or near a hip joint, and a drive source body provided on the drive shaft, the drive source body being configured to generate torque about the axial line in the lateral direction, (e) a pair of upper arms extending in a vertical direction of the wearer, the pair of upper arms being disposed respectively on both left and right sides of the upper body trunk, and attached at lower ends of the respective upper arms to one of the drive shaft or the drive source body so as to prohibit relative rotation about the axial line in the lateral direction; (f) first passive rotary shafts configured to couple upper ends of the upper arms and the upper body trunk harness respectively so as to be angularly displaceable about the axial line in the lateral direction; (g) a pair of lower arms extending in the vertical direction and disposed on both left and right sides from the lower body trunk to the thighs respectively and attached at upper ends of the respective lower arms to the other one of the drive shaft or the drive source body so as to prohibit relative rotation about the axial line in the lateral direction; (h) second passive rotary shafts configured to couple lower ends of the respective lower arms and the thigh harnesses respectively so as to be angularly displaceable about the axial line in the lateral direction; and (i) mounting means configured to mount one of midsections in a longitudinal direction of the upper arms, the drive shaft, the drive source body, and midsections in a longitudinal direction of the lower arms to the lower body trunk harness, wherein a meshed open-front vest is attached to the upper body trunk harness. (a) an upper body trunk harness to be worn and retained on an upper body trunk of a wearer; (b) a lower body trunk harness to be worn and retained on a lower body trunk of the wearer; and (c) thigh harnesses to be worn and retained on thighs of the wearer, (d) drive sources disposed on the lower body trunk on both sides in a lateral direction of the wearer, the drive sources each including a drive shaft configured to rotate about an axial line in the lateral direction at or near a hip joint, and a drive source body provided on the drive shaft, the drive source body being configured to generate torque about the axial line in the lateral direction, (e) a pair of upper arms extending in a vertical direction of the wearer, the pair of upper arms being disposed respectively on both left and right sides of the upper body trunk, and attached at lower ends of the respective upper arms to one of the drive shaft or the drive source body so as to prohibit relative rotation about the axial line in the lateral direction; (f) first passive rotary shafts configured to couple upper ends of the upper arms and the upper body trunk harness respectively so as to be angularly displaceable about the axial line in the lateral direction; (g) a pair of lower arms extending in the vertical direction and disposed on both left and right sides from the lower body trunk to the thighs respectively and attached at upper ends of the respective lower arms to the other one of the drive shaft or the drive source body so as to prohibit relative rotation about the axial line in the lateral direction; (h) second passive rotary shafts configured to couple lower ends of the respective lower arms and the thigh harnesses respectively so as to be angularly displaceable about the axial line in the lateral direction; and (i) mounting means configured to mount one of midsections in a longitudinal direction of the upper arms, the drive shaft, the drive source body, and midsections in a longitudinal direction of the lower arms to the lower body trunk harness, wherein a meshed open-front vest is attached to the upper body trunk harness.