While an insert shaft (11) disposed with a wide angle lens (15) at a top end thereof is being inserted into a hole to inspect a wall surface of the hole on the basis of an image photographed through the wide angle lens (15), gas is injected into the innermost recess of the hole along the outer circumferential surface of the insert shaft (11) by a gas injection mechanism (19). The gas injection mechanism (19) is provided with a guide (13) configured to surround a top end portion of the insert shaft (11) and form a gas injection passage (R) extending in the longitudinal direction of the insert shaft (11) between the inner circumferential surface of the guide (13) and the outer circumferential surface of the insert shaft (11), and a gas supply unit (14c) configured to supply gas to an inner space of the guide (13).