To measure the external shape and size of a long material such as an extrusion molding on an inline basis at a low cost.

This shape measuring instrument is provided with one or more light detection type sensors 2 (or laser displacement sensors) installed in a production line (such as an extrusion line) for long materials, a moving mechanism 4 for moving the sensors 2 in a direction perpendicular to the flow direction of the production line, and a position detecting means (position sensor such as displacement sensor) 3 for measuring the positions of the sensors 2. The external shape and size of a long material S to be measured is calculated based on the outputs of the sensors 2 and of the detecting means 3.