The writing instrument is equipped with the foldable boarding including plural boards foldably connected through the predetermined hinge part and other hinge parts and is capable of transforming between the flat form and the tridimensional form. As for the writing instrument like this, in the flat form, the foldable boarding doubled up along the predetermined hinge part forms a plate-like shape together with the holder which holds the writing unit. On the other hand, in the tridimensional form, the foldable boarding folded along the other hinge parts covers both principal surfaces of the holder to form a prismatic penholder by moving the predetermined hinge part to the vicinity of the specific side surface of the holder. Consequently, the writing instrument is capable of transforming from the flat form to the tridimensional form which gives a good grip feeling.