in a paper sheet discriminating device which irradiates lights of at least two or more wavelengths from a light source to a paper sheet and receives transmitting lights which transmit through said paper sheet by a photo sensor and performs the discrimination in response to light receiving signals from the photo sensor , the paper sheet discriminating device further includes reference value setting means which adjusts a light emission quantity of the light source such that the output of the photo sensor becomes a given value in a state that a reference medium is set between the light source and the photo sensor and also stores the output value of the photo sensor which directly receives light from the light source as an adjustment reference value in a memory part , and adjustment means which adjusts the light emission quantity of the light source such that the output value of the photo sensor which directly receives light from the light source is made to agree with the stored adjustment reference value .