To detect the residual quantity of a bobbin thread even during the operation of a sewing machine by controlling the operation of detecting the residual quantity of a bobbin thread according to the results of determining the rotating speed of a spindle and the turning angle range of a shuttle, in a detecting device in which a detecting rod is advanced into the shuttle through an opening of a rotating hook to detect the residual quantity of the bobbin thread.

In a control device 38 of a sewing machine, it is determined by a spindle rotating speed determining part 38C that the rotating speed of a spindle for moving up and down a needle is so low as to detect the residual quantity of a bobbin thread. Further it is determined by an opening position determining part 38A that a shuttle 34 rotated, interlocking with the spindle is in such a turning angle range as to introduce a detecting rod 31 through an opening by a driving device 32. According to the determination results, the driving device 32 is driven to detect the residual quantity of a bobbin thread through the detecting rod 31 by a bobbin thread residual quantity detecting tool 36. That is, an oscillatory wave detected by an oscillatory wave detecting element fitted to the outside of a yoke is discriminated by two kinds of comparators different in reference voltage to detect the bobbin thread.