PURPOSE: To obtain sufficient amount of reflected light from a heat sealing surface of a packaging film without receiving influence of irregular reflection on a dull surface, even if a surface of a film material of the packaging film is the dull surface by a method wherein the packaging film having a multilayer structure in which a resin coating is applied on the dull surface is used, and a right reflection rate on the surface of the resin coating layer is set to 10% or more.

CONSTITUTION: A packaging film 5 having a multi-layer structure in which a resin coating layer 9 is applied on a dull surface 8a of a film material 8 is used, and an angle of incidence θ of light from a light injector 12 with respect to a sealing surface is set so that a regular reflection coefficient (R) of the surface of the resin coating layer 9 can be 10% or more. As a result, if the surface of the film material 8 of the packaging film 5 is a dull surface 8a, sufficient amount of reflection of light can be obtained from the sealing surface of the packaging film 5 without receiving any influence of irregular reflection on the dull surface 8a. Accordingly, whether a heat sealing is good or bad can be detected correctively.