Signature data of a handwritten signature input to a signature authentication device is analyzed to determine whether to register the signature based on the characteristics of the stroke shape of the signature. If the signature registration is denied, a response message for making the signature registerable is displayed according to the way in which the signature is written. At least one of cumulative angle changes in the locus of the signature, fluctuation in the speed at which the signature was written, fluctuation in the size of the characters included in the signature, and fluctuation in the center positions of the characters included in the signature is used as the determination criterion.