A fireproof resin sash that is usable in fire zones or the like is provided, regarding which fireproof performance is easily imparted to a general non-fireproof resin sash without modification of the structure thereof. In a fireproof resin sash 1, vertical frame members 11 and 12 and lateral frame members 13 and 14 that are synthetic resin members having a plurality of hollows along the longitudinal directions thereof are combined with each other to constitute an opening body 10, and vertical rail members 21 and 22 and lateral rail members 23 and 24 are combined with each other to constitute screens 20 that support windowpanes 25. Fireproof sheets 15 and 15A made up of thermally expandable fireproof material are inserted into hollows selected from the hollows of the members so that fireproof surfaces can be formed in the direction along the glass surfaces. Preferably, the thermally expandable fireproof material is formed with material having a coefficient of volumetric expansion that increases 3 to 50 times after the material is being heated for 30 minutes under radiant heat flux of 50 kW/m2 and a stress of 0.05 kgf/cm2 or higher at a breaking point after volumetric expansion measured by a compression tester with an indenter of 0.25 cm. In addition, the fireproof sheets may be inserted together with rolled steel members or wood members into the hollows.