To provide an enlarged-diameter caisson foundation structure which is excellent in construction of a caisson foundation, and economical.

In construction of the caisson foundation, a caisson main body 11 having a circular cross section is driven into the ground until a front end 11b is embedded in a support layer 8, and a head portion 11a of the main body bears a bridge pier 10. On the periphery of the caisson main body 11, a cylindrical steel sheet pile wall 12a is driven into the ground 7 until it reaches a predetermined depth. Next, earth in a gap between the caisson main body 11 and the steel sheet pile wall 12 is improved by high-pressure jet mixing or the like to form a hardened layer 13, and the caisson main body 11, the steel sheet pile wall 12a, and the hardened layer 13 are unitedly connected together, to thereby form an enlarged-diameter caisson 16.