Surface modification device forms a discharge area E1 between a discharge electrode 6 and a counter electrode 4, supplies substitution gas to the discharge area E1, and modifies the surface of the base material to be processed. The surface modification device comprises; a slit-shaped substitution gas passage; and cover members 7, 8 that form curtain passages 22, 23 in spaces facing the discharge electrode. While the substitution gas is being supplied to the discharge area E1, gas injected from the curtain passages 22, 23 prevent the inflow of an entrained flow a and the outflows b1, b2 of the substitution gas, thereby maintaining the concentration of substitution gas inside the discharge area E1.