In an apparatus for detecting and displaying a residual quantity of lower or under thread (12) of a sewing machine, an under-thread bobbin (10) is provided with transparent flange portions (10a) permitting the light rays emitted from a light emitting device of a photosensor (19) to pass through the flange portions (10a ) of the bobbin (10). The light rays having passed through the bobbin (10) are then reflected by a reflecting means or mirror (21) affixed to a back surface of a bed of the sewing machine to impinge on a light receiving device of the photosensor (19) so that the photosensor (19) detects the residual quantity of the lower thread (12) of the sewing machine. In the photosensor (19), the light emitting device is integrally formed with the light receiving device to save space.