PURPOSE: To detect the foreign matters in a hollow molding by judging the presence or absence of foreign matters in a parison by a method in which laser is irradiated onto a parison immediately after being extruded, and the transmitted light is received by a light receiver and the amount is electrically processed.

CONSTITUTION: A low-density transparent polyethylene mixed and melted by rotation of a screw 2 in a heating cylinder 1 is extruded into a parison 4 through a head 3. The parison at this moment is extremely excellent in transparency due to being heated over the melting point. The transparent parison 4 immediately after being extruded the said polyethylene is exposed to irradiation of laser from an optional direction with a laser projector 7 using a He-Co or Ar gas laser suitable for the discrimination of color dificiency, e.g., red color deficiency originated from foreign matters. The transmitted light from the parison is received by a receiver 6 and the amount is electrically processed to detect the foreign matters in the parison.