To provide a thermal insulation member for forming a ventilation layer of a building and thermal insulation structure of the building using the same capable of simplifying the construction work by forming the ventilation layer and a thermal insulation layer at one time and improving construction accuracy at all times.

A thermal insulation member 15 for forming ventilation layer made of polystyrene foam is provided with a recessed part 16 for forming ventilation layer and a pressure- contact part 17 in contact with a sheathing roof board 13 while it is brought into pressure- contact with common rafters 14 on both sides. The pressure-contact part 17 is provided with a slit part 18 to obtain elasticity between an end part side of the pressure-contact part 17 and a main body side across the slit part 18. The common refuters 14 are attached to a lower face of the roof board plate 13 at a predetermined interval, the thermal insulation member 15 is fitted between the common rafters 14 in such a way that the recessed part faces a roof board plate 13 side and the pressure-contact part 17 comes into pressure-contact with the common refter 14 and comes into contact with the roof board plate 13, and a thermal insulation layer 21 made of foaming urethane resin is formed on lower face sides of the thermal insulation member 15 and the common rafter 14 by spraying it.