A reaction curable composition according to an aspect of the present disclosure contains a reactive component (A) and a filler (B). The filler (B) contains an antireflection filler (B1). The antireflection filler (B1) has an average particle size of greater than or equal to 0.8 μm and less than or equal to 10 μm, and particles of the antireflection filler (B1) each have a surface having a plurality of projections. A mean diameter of the plurality of projections is greater than or equal to 100 nm and less than or equal to 500 nm.