To improve the workability of forming an inside room.

This inside room forming method for forming a new room space R in an existing room inside 1a by assembling multiple room forming panel members M includes that multiple unit frame materials 3 (3a and 3b) and a finish surface material are prepared in the existing room 1a as the room forming panel members M and after the multiple unit frame materials (3a and 3b) are assembled in the existing room 1a, a part facing to the side of the new room space R of the assembled unit frame materials 3 (3a and 3b) is lined with the finish surface material 4 so as to form the new room space R.