A method is disclosed for authenticating, by a processor that controls a parent device, a child device includes: authenticating the child device by making a comparison between a value obtained by operating, for a first response value, a third transform function, which is decided based on a number of a difference between the value set in an authentication chip of the parent device and the value set in an authentication chip of the child device, and the second response value, wherein a first and a second response values are obtained by operating a first and a second transform functions for output values generated by operating an encryption function for performing encryption for secret keys in authentication chips of the parent device and the child device, respectively.