An assembling block includes block plates having square and equilateral triangular plan shapes, respectively, and a joint for connecting the block plates together. Each of the block plates includes seat portions provided along the respective sides of the block plate, and retaining pieces extending from the center of the block plate to the outer edge thereof and spaced from the seat portions in the thickness direction. The joint comprises a plate member made of a flexible and bendable material, whereby when the joint is inserted between two of the seat portions and one of the retaining pieces of each block plate, the joint engages the seat portions with engaging protrusions formed on the seat portions fitted in engaging holes formed in the joint, thereby coupling the block plates together.