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$.$C$^$0.9294/, and between the number of users(P) showed Q=2${ imes}$10$^$-8/ $.$P$^2$- 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${ imes}$10$^$-6/$.$Q 0-0.2266$.$Q+29,895, according to the elimination of BOD(E) showed S=6${ imes}$10$^$-5/$.$E$^2$-0.6717$.$E + 27,744, according to the annual amount of sale showed S=0.0005 C$^2$-4.8013$.$C + 35,118, with the number, of persons(P) using the service area showed S= 2${ imes}$10$^$-8/ $.$P$^2$- 0.046$.$P + 48,803.