This invention relates to a sound insulating building block that insulates, absorbs or irregularly reflects incident sound waves. The block contains a cover therein. The cover contains a sound absorbing plate formed into a plate shape of mineral fibers such as glass wool or asbestos or other ordinary fibers additionally attached to the mineral fibers. The sound absorbing plate is inserted into the cover in tight contact with the inside of the cover and the cover is then received into an open block made of mineral matter such as concrete and having side plates on the circumference. The cover is provided on its facing panel with slits or holes designed to face the open portion of the block.