A sensor including a tubular casing having an open rear end; and a first elastic member having a through hole so as to introduce ambient air into the casing and disposed so as to block the rear end opening of the casing. The sensor further includes: a holding inner tube that closely contacts an inner peripheral surface of the through hole; a ventilation filter that has permeability and a waterproofing property, the ventilation filter including a filter blocking portion that blocks a rear end opening of the holding inner tube, and a filter tubular portion connected to the filter blocking portion and surrounding at least a portion of an outer peripheral surface of a rear end portion of the holding inner tube in the peripheral direction; and a holding outer tube that includes a ventilation hole, an outer tube blocking portion covering the filter blocking portion, and an outer tube tubular portion connected to the outer tube blocking portion and surrounding at least a portion of the filter tubular portion in the peripheral direction.