To provide a flexible polyurethane slab foam having high mechanical properties.

This flexible polyurethane slab foam is obtained by reacting a strength-improving agent (A) having a specific structure represented by general formula (I) with an organic polyisocyanate component (D) in the presence of a foaming agent, a catalyst and a foam stabilizer and has air permeability of 10-150 mL/cm²/sec. In general formula (I), R1 represents a residue excluding one active hydrogen from an active hydrogen-containing compound, a plurality of R1s may be the same or different from each other; Y represents a residue excluding a carboxy group from three or more hydric aromatic polycarboxylic acid (C), the aromatic ring of Y is composed of carbon atoms. The substituent of the aromatic ring may be a hydrogen atom or other substituents, provided that at least one substituent is a hydrogen atom; a is an integer satisfying: 2≤a≤((the number of aromatic ring substituent)-2); Z is a residue excluding m pieces of active hydrogen from m or more values of active hydrogen-containing compound; and m denotes an integer of 1 to 10.