A substrate processing apparatus 30 includes a first holding unit 200 configured to hold a processing target substrate W; a second holding unit 300 disposed to face the first holding unit 200 and configured to hold a support substrate S; and an ultraviolet irradiation unit 400 configured to irradiate an ultraviolet ray to an adhesive G provided between the processing target substrate W and the support substrate S. Each of the support substrate S and the second holding unit 300 is made of an ultraviolet transmissive material. An electrode 320 configured to electrostatically attract the support substrate S is provided within the second holding unit 300. A diffusion layer 330 configured to diffuse a transmission direction of the ultraviolet ray is provided at a position closer to the support substrate S between the support substrate S and the electrode 320 within the second holding unit 300.