To reduce temporary structures on the sea to shorten a construction period on the sea regarding a foundation slab of a sea berth structure.

This foundation slab structure of the sea berth structure is provided with a half precast concrete slab 1 having a void space 2 surrounding pile heads of a plurality of foundation piles 6 so that peripheries of the pile heads are arranged in a state of being located in the void space 2, and cast-in-place concrete 11 placed around the pile heads in the void space 2. Beam members 3 are further provided joined to the pile heads and extended over the void space 2. The number of assemblies of the temporary structures on the sea for constructing the foundation slab can thereby be considerably reduced, and the construction period on the sea can be shortened.