A substrate processing apparatus includes: a rotary cylindrical member (DR) that includes a cylindrical supporting surface curved with a constant radius from a predetermined center line (AX2) and that feeds a substrate (P) in a length direction of the substrate; a processing mechanism that performs a predetermined process on the substrate at a specific position (PA, EL2) of a part of the substrate; a scale member (SD) that rotates about the center line along with the rotary cylindrical member so as to measure a displacement in a circumferential direction of the supporting surface of the rotary cylindrical member or a displacement in a direction of the center line of the rotary cylindrical member and that includes a scale portion (GP) carved in a ring shape; and a reading mechanism (EN1, EN2) that faces the scale portion, that is disposed in substantially a same direction as the specific position when viewed from the center line, and that reads the scale portion.