A construction of a floor slab bridge includes a plurality of columnar H-shaped steels each disposed between adjacent bridge legs and arranged in side-by-side relation with an end face of a lower flange abutted with a corresponding end face of the adjacent columnar H-shaped steel. A lower concrete layer is formed by placing concrete in a space defined between the upper and lower flanges and between adjacent web plates through a concrete inlet port formed between the adjacent upper flange, and an upper concrete layer is formed by placing concrete on the upper flange. An iron reinforcement is horizontally disposed on the upper flanges, and an iron reinforcement is suspended in the space from the horizontal iron reinforcement through the concrete inlet port.