To provide a support-to-horizontal member connection device capable of securing a support and a horizontal member with almost similar strength to welding without deteriorating strength of the support, capable of combining horizontal members of free form to be connected at a free position in a height direction, and having design freedom in combined supports for construction, entrance poles, or combined poles in an event hall in which a specified number of supports are combined with horizontal members to form a triangle, a square, a pentagon, or a hexagon as seen in a plan view.

Plural internally enlarged grooves 11 extended in a length direction are provided on an outer circumferential surface of each support 1, and a first hook part 21 is provided at a connection end part 2a of each horizontal member 2. A first hook part 21 at the connection end part 2a of the horizontal member 2 provided between the supports 1 is fitted in one internally enlarged groove 11a in the support 1 to be hooked on an inner protruded part 12 at one side part of its opening, and they are fastened by a coupling tool 30 having a second hook part 31, a bolt 40 and a nut 41.