There is described an induction sealing device for heat sealing packaging material for producing sealed packages of pourable food product and fed in a first direction. A defective information storing means for storing a defect point correlation with a portion of an education material is provided. When it is assumed that the refractive index of a base material of the first and second prisms is Np, the resultant refractive index of the parallel flat layers other than the other parallel flat layer of the at least two parallel flat layers is Na, and the refractive index of the other parallel flat layer of the at least two parallel flat layers is Nb, \xd7<0 is satisfied. A flaw detection circuit is capacitively coupled to the electrical conductor and receives a signal indicative of a voltage signal on the electrical conductor.