Lightguide plate and lightguide plate manufacturing method

A film-shaped lightguide plate having high light entrance efficiency, and which is easy to make, has a lightguide plate body having an upper surface as a light-exiting surface, a lower surface opposite to the upper surface, and a peripheral side surface extending between the respective peripheral edges of the upper and lower surfaces. A circumferential part of the peripheral side surface forms a light-entrance surface that faces a light source. The lightguide plate further has an additional lightguide portion formed on at least one of the upper and lower surfaces of the lightguide plate body at the light-entrance surface side. The additional lightguide portion has a light-entrance surface facing the light source, together with the light-entrance surface of the lightguide plate body. The additional lightguide portion receives light from the light source through its light-entrance surface and guides the light into the lightguide plate body.