To provide a bobbin thread residual amount detector for a sewing machine which is capable of more suitably detecting the residual amount of bobbin thread.

The bobbin thread residual amount detector 100 for the sewing machine includes: an illumination 2 for making light be incident inside a transparent shaft part 1a of the bobbin 1; a line sensor 3 for detecting the luminance of the entire range W where a bobbin thread T is to be wounded around in the axial direction of the shaft part 1a of the bobbin 1; a CPU 11 for determining the residual amount of the bobbin thread from the luminance detected by the line sensor 3; and an operation panel 20 for displaying an image for reporting that it is determined that the residual amount of the bobbin thread has decreased.