Foreign substance inspection method and foreign substance inspection apparatus

A foreign substance inspection method for inspecting presence or absence of a foreign substance embedded in a tubular resin molded product and having a dimension larger than a predetermined threshold, the foreign substance inspection method comprising:

• preparing, as the resin molded product, a resin molded product made of nylon that does not include color pigment;
• a first measurement step of emitting light to a first peripheral surface of the resin molded product from a first light source and measuring a dimension of a first projection image, of an embedded foreign substance, that is projected on a second peripheral surface, the first light source being placed on either one of inside and outside of the resin molded product in a tubular radial direction;
• a second measurement step of emitting light to the second peripheral surface of the resin molded product from a second light source and measuring a dimension of a second projection image, of the embedded foreign substance, that is projected on the first peripheral surface, the second light source being placed on the other one of the inside and the outside of the resin molded product in the tubular radial direction;
• and
• a calculation step of calculating a dimension of the embedded foreign substance based on a proportional relationship between a dimension of each of the projection images projected by the light sources to a distance from a corresponding one of the light sources to the each of the projection images and a dimension of the embedded foreign substance to a distance from the corresponding one of the light sources to the embedded foreign substance.