An auxiliary tunneling apparatus includes a reaction force receiver and first and second split components. In the excavation of a second tunnel by a boring machine, the reaction force receiver forms a replacement face of a side wall of the second tunnel on a first tunnel side where the first and second tunnels intersect each other, and a gripper of the boring machine pushes against the replacement face. The first and second split components are installed to push against the side wall of the first tunnel, support the reaction force receiver within the first tunnel, and move back and forth with respect to the side wall of the first tunnel.