A box is made of structural boards having a face plate (2) including a main body portion (2c) that is substantially in one plane and a peripheral portion (2a) surrounding the main body. The boards further include a base plate (3) having integrally formed protrusions. The lower surface of the face plate (2) is attached to and supported on top of the protrusions. A stiffener (4) connects the peripheral portion (2a) of the face plate (2) and the base plate (3) together. At least a part of the stiffener (4) is fixedly connected to the base plate (3), and at least another part of the stiffener (4) is fixedly connected to at least a part of a lower surface of the peripheral portion (2a) of the face plate.