A paper thickness detecting apparatus 300 includes a first sensor 301 and a second sensor 302 in which the electrostatic capacitance changes when a piece of paper 304 such as a bank note or a piece of copying paper is inserted, and a thickness detecting unit 303 for detecting the thickness of the piece of paper 304 from the signals obtained from the respective first sensor 301 and the second sensor 302, in order to provide a paper thickness detecting apparatus that can reliably detect a foreign object adhered to the paper.