A portable terminal device authentication system, comprising: an in-vehicle device including, an in-vehicle side transmission unit configured to transmit a request signal, an in-vehicle side reception unit configured to receive an answer signal, and an in-vehicle side control unit configured to cause the in-vehicle side transmission unit to transmit the request signal and determine validity of the answer signal that is received by the in-vehicle side reception unit; and a portable terminal device including, a portable side reception unit configured to receive the request signal, a portable side transmission unit configured to transmit the answer signal, and a portable side control unit configured to control the portable side transmission unit to transmit the answer signal in response to reception of the request signal by the portable side reception unit, characterized in that the in-vehicle side control unit generates a plurality of instruction signals which are different in a duty ratio, and supplies the instruction signals which are generated to the in-vehicle side transmission unit in correspondence with the request signal, the in-vehicle side transmission unit generates an authentication signal with signal intensity corresponding to the duty ratio of the plurality of instruction signals which are supplied, and transmits the request signal including the authentication signal to the portable terminal device, the portable side control unit performs processing based on an intensity value of the authentication signal which is included in the request signal that is received by the portable side reception unit, and the in-vehicle side control unit or the portable side control unit determines validity of the in-vehicle device or the portable terminal device on the basis of a processing result of the portable side control unit. an in-vehicle device including, an in-vehicle side transmission unit configured to transmit a request signal, an in-vehicle side reception unit configured to receive an answer signal, and an in-vehicle side control unit configured to cause the in-vehicle side transmission unit to transmit the request signal and determine validity of the answer signal that is received by the in-vehicle side reception unit; and a portable terminal device including, a portable side reception unit configured to receive the request signal, a portable side transmission unit configured to transmit the answer signal, and a portable side control unit configured to control the portable side transmission unit to transmit the answer signal in response to reception of the request signal by the portable side reception unit, characterized in that the in-vehicle side control unit generates a plurality of instruction signals which are different in a duty ratio, and supplies the instruction signals which are generated to the in-vehicle side transmission unit in correspondence with the request signal, the in-vehicle side transmission unit generates an authentication signal with signal intensity corresponding to the duty ratio of the plurality of instruction signals which are supplied, and transmits the request signal including the authentication signal to the portable terminal device, the portable side control unit performs processing based on an intensity value of the authentication signal which is included in the request signal that is received by the portable side reception unit, and the in-vehicle side control unit or the portable side control unit determines validity of the in-vehicle device or the portable terminal device on the basis of a processing result of the portable side control unit.