PURPOSE: To control blinking of each ring-shaped light source speedily, transmit obtained image information speedily, and achieve a high-resolution detection.

CONSTITUTION: Ring-shaped light sources 20a,..., 20c are placed in three steps at the upper part of a part to be inspected, at the same time a black and white camera 22 is laid out, each light source is allowed to blink at different timings successively, the image information obtained at each lighting timing is stored at corresponding frame memories 26a,..., 26, and then it is judged whether soldering is proper or not according to the image characteristics. In each ring- shaped light source, a number of light emitting diodes 32 are aligned in a downward channel-shaped cover material 30. An interlace scanning system is adopted. the ring-shaped light source lights up only at a half cycle where odd/even image pick-up times are superposed for exposing to the black and white camera, and then the odd/even image pick-up signal is transferred to a frame memory during a next one cycle.