To provide a bobbin for reliably supplying an upper thread in a sewing machine.

A lower thread bobbin winder (1) is provided with a bobbin (22) where a plurality of slits (224 and 225) are formed in the outer circumferential edge of a flange at an upper part (upper flange 222) and a thread guard member (48) for abutting the upper thread onto the outer circumferential edge part of the upper flange. An upper slope (225b) inclining upward from the lower surface of the flange is formed in a side edge (225a) at a rear side concerning the bobbin rotation direction of one slit and a lower slope (224b) inclining downward from the upper surface of the flange is formed in the side edge (224a) at the rear side concerning the bobbin rotary direction of one slit. Thus the upper thread is caught into the slit when the upper thread is positioned to be even a little lower than the upper end part of he upward slope.