A primer composition used for applying a radical-curable resin-based waterproof material onto a concrete floor slab of an existing or newly established road bridge, is a troublesome application. For example, it is a problem in that a large machine is required for application thereof. The primer composition is a resin composition containing a urethane (meth)acrylate resin, (B) a polymerizable unsaturated monomer containing a hydroxyalkyl (meth)acrylate (B-1) and another polymerizable unsaturated monomer (B-2), (C) a curing accelerator, (D) a paraffin wax, and (E) a drying aid, in which the above components satisfy a specific molar ratio and specific mass ratios. The primer composition has a good crack-load-following capability, blistering resistance, adhesiveness, and bending resistance. Also provided is a waterproof floor slab structure using the primer composition and a method for waterproofing a floor slab.