To effectively use short materials by combining by applying work for joining to an end part of a bar-shaped short material and in the longitudinal direction by using small diameter wooden thinning timber and the short materials appearing in large numbers in a lumbering process, and combining without using a nail and an adhesive up to a large structure such as a storeroom and a log house in addition to a grating-like structure, a rack, a handrail and a planter.

This device is constituted so that a 1/4 space of the residual cross-sectional area except for the cross-sectional area of one fixing key material 5 having a square cross section arranged in a central part of a cross can be used as a space for joining one member from the cross-sectional area of a square ABCD for joining for members. Four block materials are joined and fixed in a cross shape Fig.9 between the mutually adjacent members by inserting the one fixing key material 5 having a quadrangular or round cross section into the center of the cross by combining the four block materials formed by respectively arranging an engaging projection part and an engaging groove 6 such as a cross-sectional shape part for joining becomes the same shape and the same area in both four members.