To provide a building constructing method using foam synthetic resin plates, in which foam synthetic resin plates such as styrene foam plates are used for all wall panels and roof panels constituting a building such as a housing, the structure of a foundation part and a panel joint method are simplified, and thereby the building can be rapidly and easily constructed at low cost.

A U-shaped foundation part having panel fitting grooves for fixing wall panels made of foam synthetic resin is formed on a top face of a foundation concrete; wall panels having a standard size dimension are inserted into and bonded or fixed in a sandwiching state to the panel fitting grooves of the foundation part, while the lateral faces of the wall panels are bonded and fixed to one another to construct an outer wall of the building; and furthermore, roof panels made of a foam synthetic resin are bonded and fixed on the top face of the outer wall. In this way, the building having a square space inside is formed.