To obtain a matt foundation in which placement of ready-mixed concrete can be performed from the inside of a formwork of a rising part when forming the matt foundation, and a placement joint surface is not formed between a lower end of the rising part and a slab.

In right and left longitudinal channel walls 25 of a plurality of connected lower channel external bodies 1b, a plurality of reinforcing-bar longitudinally long channel through holes 26 in a longitudinally long shape with an upper end side as an open channel hole are opened using predetermined dimension intervals. Slab reinforcing-bars 27 are inserted into the reinforcing-bar longitudinally long channel through holes 26. Within recessed channels of the lower channel external bodies 1b, a plurality of upper cover fulcrums 29 with open width fixed large channels are erected orthogonally with a channel flow direction. The lower channel external bodies 1b and the fulcrums 29 are fixed to the ground by tie rods 52. On a reinforcing-bar placement channel 31 within the fulcrum 29, a rising part reinforcing-bar 33 is disposed and supported by a removable support steel rod 53. On the fulcrum 29, a rising part formwork 32 to be erected on an upper cover channel external body 2 is supported which should be placed on the fulcrum 29, and the rising part reinforcing-bar 33 and the slab reinforcing-bar 27 are connected. After placement of ready-mixed concrete, the removable support steel rod 53 is removed.