Accumulation-type thread feeder including: a body, which bears a rotary drum on which turns of thread from a spool are wound; a tension sensor for detecting value of tension of exiting thread and a detector for detecting thread quantity accumulated on the drum; a light-reflecting element on the drum or functionally associated therewith, such light generated by light a generator borne by a support arranged alongside the drum, the support associated with a detector to detect light reflected by the reflecting element, the reflected light varying as a function of the quantity of thread wound on the rotary drum, the detection allowing detection of such thread quantity. The detector is a light-sensitive member directly receiving the light reflected by the reflecting element along the entire surface of the drum, the member allowing determination of thread density on the drum.