A structure for attaching a blocking member displaces the blocking member from an insertion position where clamping pawl parts are arranged in a notch part to a fixation position where an opening edge portion is clamped in clearances between the clamping pawl parts and a main body part of the blocking member, thereby to attach the blocking member to an attachment object member. A projecting part that projects toward the blocking member is formed around an opening. At least a part of the projecting part is a side surface projecting part that projects along an outer peripheral side of a side surface of the blocking member when the blocking member is displaced to the fixation position. At least one of the clamping pawl parts is a low-load clamping pawl part. A clearance size between the low-load clamping pawl part and a main body part of the blocking member is provided to be larger than a clearance size between another clamping pawl part and the main body part of the blocking member.