This study aimed to obtain the relative formula with the unit treatment cost according to the treatment of a sewage plant in the service area under highway. The following results were obtained. The correlative formula connected to amount of sewage(Q)generation was as follows ; between an annual amount of sale(C) showed Q=19.113·C0.9294, and between the number of users(P) showed Q=2×10-8·P2 - 0.0298·P + 75,666. The correlative formula connected to the treatment cost was as follows ; according to the amount of sewage generation showed S= 3×10-6·Q - 0.2266·Q + 29,895, according to the elimination of BOD(E) showed S= 6×10-5·E2 - 0.6717·E + 27,744, accoding to the annual amount of sale showed S=0.0005·C2 - 4.8013·C + 35,118, with the number of persons(P) using the service area showed S= 2×10-8·P2 - 0.046·P + 48,803.