Abstract: A toy building block set includes a socket block piece and at least one of first and second plug block pieces. The socket block piece has a socket wall body and at least one engaging hole of a regular polygonal shape. Each of the first and second plug block pieces has a plug body and at least one polygonal insert plug configured to be inserted into and in frictional engagement with the engaging hole with a first frictional force to permit rotation of the plug block piece relative to the socket block piece by a forcible torque and to keep a predetermined angle of the plug block piece relative to the socket block piece when the forcible torque is removed. The plug body is partially inserted into and in frictional engagement with the engaging hole with a second frictional force larger than the first frictional force.