A bill acceptor capable of reducing light pollution, avoiding errors and faults and enhancing detection accuracy and reliability is disclosed to include a housing defining a bill slot in a face panel thereof, a bill-receiving unit accommodated in the housing and including a holder base defining therein a bill passage in communication with the bill slot, a control module, a transmission mechanism and a recognition circuit assembly, the recognition circuit assembly including a sampling and validating module consisting of a plurality of light emitters and light sensors and controllable by the control module to validate the authenticity and face value of an inserted bill, and privacy filters mounted in a light path in the bill-receiving unit between the bill passage and the light sensors to filtrate external light sources and to let the light emitted by the light emitters pass toward the light sensors.