Air embolism can be a lethal complication of surgical procedures during which venous pressure at the site of surgery is sub-atmospheric or air is forced under pressure into a body cavity. To solve the problem, we developed the air detector using relative dielectric constant change, which is expected to be used broadly in industrial circles. We designed a detection circuit of sensing scheme. In experiments with a mock system, the proposed system showed a signal difference depending on the amount of air in the tygon tube of the mock system.