To provide a panel reinforcing structure for a panel shutter capable of increasing the strength of a panel against a negative pressure with a simple reinforcing structure.

A panel material 2 for the panel shutter 1 comprises a long surface material 5, a plurality of long lateral frame member materials 6 and 7 installed integrally with the surface material 5 along the longitudinal direction thereof, and a plurality of vertical frame member materials 8, etc., installed integrally with the surface material 5 in an orthogonal state to the lateral frame member materials 6 and 7. A hollow reinforced side part 7b projected to an indoor side only in the generally vertical direction with respect to a frame member body 7a and the panel material 2, is formed integrally with at least one of the lateral frame member materials 5 and 6.