An object of the present invention is to provide an element connecting apparatus with which the position of a core of each element in an optical transmission module in a short time with high accuracy. To attain the object, in a core position detection method according to the present invention, upon detecting a core of a PLC chip, parallel light is made incident on the output side end face of the PLC chip, so that the core position is detected based on an image of the input side end face obtained with the parallel light. In addition, the core position is detected by an image of the output side end face that is obtained while parallel light is made incident on the input side end face.