The present invention relates to a foundation structure vertically installed on the ground, and comprising: an upper support layer 10 formed on the ground in the vertical direction; and a lower support layer 20 formed so as to extend downward from the upper support layer 10 and such that the width thereof is less than that of the upper support layer 10. The disclosed upper support layer 10 and the lower support layer 20 are structures formed from soil solidified by means of feeding and mixing an earth and soil-solidifying agent therein, and therefore are efficient and prevent overload due to boring equipment.