A pair of regular hexahedral blocks connectable to each other, wherein each block of the pair of regular hexahedral blocks comprises: a protrusion for connecting the respective regular hexahedral blocks to each other, characterised in that the protrusion is formed on a respective first surface of the respective regular hexahedral blocks at the center of one quarter section of the respective first surface, three recessed portions (B12, B13, B14), wherein said three recessed portions are formed at the center of the three quarter sections of the respective first surface of the respective regular hexahedral blocks where the protrusion is not formed, and at least one further recessed portion (B21, B41, B51, B61), wherein said one further recessed portion is provided at the center of one quarter section of each of the respective second, fourth, fifth and sixth surfaces (B2, B4, B5, B6) of the respective regular hexahedral blocks perpendicular to the respective first surface and wherein at least one further recessed portion is provided at the center of one quarter section of the respective third surface of the respective regular hexahedral blocks parallel to the respective first surface, wherein in each of the respective second, fourth, fifth, and sixth surfaces (B2, B4, B5, B6), recessed portions (B21, B22, B41, B42, B51, B52, B61, B62) are provided at the center of two respective diagonal quarter sections of the surface, and the remaining quarter sections are provided with no protrusion or recessed portion, and wherein recessed portions are provided in the center of all the four quarter sections of the respective third surface,. wherein the respective recessed portions (B21, B22, B41, B42, B51, B52, B61, B62) are provided diagonally in the same direction in each of the respective second, fourth, fifth, and sixth surfaces (B2, B4, B5, B6), and wherein the protrusion has a square cross-sectional shape, and wherein at least one recessed portion includes four inner wall surfaces and has a cross-sectional shape into which a protrusion of square cross-sectional shape is fittable. a protrusion for connecting the respective regular hexahedral blocks to each other, characterised in that the protrusion is formed on a respective first surface of the respective regular hexahedral blocks at the center of one quarter section of the respective first surface, three recessed portions (B12, B13, B14), wherein said three recessed portions are formed at the center of the three quarter sections of the respective first surface of the respective regular hexahedral blocks where the protrusion is not formed, and at least one further recessed portion (B21, B41, B51, B61), wherein said one further recessed portion is provided at the center of one quarter section of each of the respective second, fourth, fifth and sixth surfaces (B2, B4, B5, B6) of the respective regular hexahedral blocks perpendicular to the respective first surface and wherein at least one further recessed portion is provided at the center of one quarter section of the respective third surface of the respective regular hexahedral blocks parallel to the respective first surface, wherein in each of the respective second, fourth, fifth, and sixth surfaces (B2, B4, B5, B6), recessed portions (B21, B22, B41, B42, B51, B52, B61, B62) are provided at the center of two respective diagonal quarter sections of the surface, and the remaining quarter sections are provided with no protrusion or recessed portion, and wherein recessed portions are provided in the center of all the four quarter sections of the respective third surface,. wherein the respective recessed portions (B21, B22, B41, B42, B51, B52, B61, B62) are provided diagonally in the same direction in each of the respective second, fourth, fifth, and sixth surfaces (B2, B4, B5, B6), and wherein the protrusion has a square cross-sectional shape, and wherein at least one recessed portion includes four inner wall surfaces and has a cross-sectional shape into which a protrusion of square cross-sectional shape is fittable.