PURPOSE: To enable accurate detection of a bobbin thread remaining quantity by detecting the decrement in the bobbin thread remaining quantity when the rotational direction of a bobbin changes.

CONSTITUTION: When light emitting elements 95, 97 are operated, spot beams are projected onto a rotary display plate 83 exposed from a window part 77. Light receiving elements 99, 101 receive individually reflected light from the rotary display plate 83 so that detection signals corresponding to the respective entering light quantities are issured. Consequently, the signal output of light receiving elements 99, 101 facing a colored portion 89 is lowered. By mutually comparing signal outputs of the respective light receiving elements 99, 101 in an electronic control circuit 103, the rotational direction of a bobbin 3 can be detected. That is, the remaining quantity of bobbin thread 5 can be detected through the rotational direction of the bobbin 3. Accordingly, the remaining quantity of bobbin thread 5 can be detected accurately regardless of its thickness, color, and material.