Form system for construction of underground slab and method for constructing underground slab and breast wall using the same

A structure for constructing a slab of an underground structure and a construction method of the slab without a wall girder and an underground retaining wall thereby are provided to prevent leak by reducing the joint surface of the underground retaining wall, and to cut down construction cost by omitting the wall girder in constructing the slab. A slab construction structure(104) includes a support girder(110) inserted in an underground retaining wall space and supported by a girder support unit installed to an H-pile, an inner reinforcing beam(113) fixed to the support girder by a gusset plate, a deck plate(116) supported on the support girder and the inner reinforcing beam and installed in a slab, a form unit(117) for forming a slab boundary along an interface of the slab and the underground retaining wall on the support girder and the inner reinforcing beam, and a support frame(120) connected and installed between the retaining wall and the slab to transmit force between the retaining wall and the slab. The support frame is composed of plural wale support frames(121) arranged continuously between the retaining wall and the slab. The wale support frame comprises a wale(122) horizontally supported on an upper bracket by welding and provided with a backfill part transmitting earth pressure, and plural struts(123) welded into the wale and connected to the slab in an upper part of the inner reinforcing beam and the support girder with the form unit.