To provide form blocks for a wall that can be simply constructed at a job site by hardly receiving restriction of a construction condition, and to provide a wall structure and a method for constructing a subterranean structure.

The form blocks 1 for the wall are vertically piled along a steel sheet pile wall 103 to be horizontally arranged. A wall body 12 is formed to fill a space between the blocks 1 and the steel sheet pile wall 103 with concrete 108. The blocks 1 are provided with an outer plate part 2 forming one side of the wall body 12 and arm parts 3 and 3 projected in the inner face of the outer plate part 2. The blocks 1 are vertically piled along one side of the steel sheet pile wall 103 to be horizontally arranged, and the wall body 12 composed by filling the space between the outer plate part 2 of the blocks 1 and the steel sheet pile wall 103 with the cast-in-place concrete 108 can be constructed at the job site.