To provide a checking tool playing a role of eliminating forgetfulness of fastening and lack of fastening and capable of reducing labor because of recent enforcement of a house quality security promoting raw in April, 2000 by an increase in defective houses by ommision work, serious consideration to a check on whether or not a structure is surely connected and fixed by bolts and nuts, the frequent occurrence of fastening forgotten or insufficently fastened places at work in connecting work of the structure by using a large quantity of bolts and nuts and the necessity of the checking tool for completely connecting and fixing these bolts and nuts.

The truncated cone-shaped checking tool is installed between a washer inserted into a shaft of the bolts and the structure, and the function is imparted for indicating execution of the sure connection by making the push- expanded truncated cone-shaped checking tool appear on the edge outside of the washer when surely fastening, connecting and fixing the connecting structure by the bolts and nuts. This purpose is solved by making the checking tool not appear on the washer side edge when the fastening is insufficient or appear in the expected size or less.