To provide a block having a simple shape by itself with which various three-dimensional structures can be obtained by devising connection states, the invention is directed to a regular hexahedral block connectable with at least another block by fitting a protrusion provided on a surface of one of the blocks into a recessed portion provided in a surface of the other block, in which one protrusion having a square cross-sectional shape is formed at the center in one quarter section of a surface of the regular hexahedron, and in which a recessed portion into which the protrusion is fittable is formed at the center in each of two to four quarter sections of any other three surfaces of the regular hexahedron.