A control system for a vehicle, the control system comprising: a portable authentication terminal configured to be able to authenticate a user who carries the portable authentication terminal; and a vehicle configured to communicate with the portable authentication terminal, wherein: the vehicle includes a sound generation unit configured to generate a predetermined sound within a vehicle cabin, an information reception unit configured to receive information transmitted from the portable authentication terminal, and a determination unit configured to determine whether or not the portable authentication terminal is within the vehicle cabin based on the information received by the information reception unit; the portable authentication terminal includes a sound collection unit configured to collect a sound, and an information transmission unit configured to transmit information based on the sound collected by the sound collection unit to the vehicle; and the determination unit is configured to determine whether or not the portable authentication terminal is within the vehicle cabin based on whether or not the information received by the information reception unit is information based on the sound generated by the sound generation unit. a portable authentication terminal configured to be able to authenticate a user who carries the portable authentication terminal; and a vehicle configured to communicate with the portable authentication terminal, wherein: the vehicle includes a sound generation unit configured to generate a predetermined sound within a vehicle cabin, an information reception unit configured to receive information transmitted from the portable authentication terminal, and a determination unit configured to determine whether or not the portable authentication terminal is within the vehicle cabin based on the information received by the information reception unit; the portable authentication terminal includes a sound collection unit configured to collect a sound, and an information transmission unit configured to transmit information based on the sound collected by the sound collection unit to the vehicle; and the determination unit is configured to determine whether or not the portable authentication terminal is within the vehicle cabin based on whether or not the information received by the information reception unit is information based on the sound generated by the sound generation unit. the vehicle includes a sound generation unit configured to generate a predetermined sound within a vehicle cabin, an information reception unit configured to receive information transmitted from the portable authentication terminal, and a determination unit configured to determine whether or not the portable authentication terminal is within the vehicle cabin based on the information received by the information reception unit; a sound generation unit configured to generate a predetermined sound within a vehicle cabin, an information reception unit configured to receive information transmitted from the portable authentication terminal, and a determination unit configured to determine whether or not the portable authentication terminal is within the vehicle cabin based on the information received by the information reception unit; the portable authentication terminal includes a sound collection unit configured to collect a sound, and an information transmission unit configured to transmit information based on the sound collected by the sound collection unit to the vehicle; and a sound collection unit configured to collect a sound, and an information transmission unit configured to transmit information based on the sound collected by the sound collection unit to the vehicle; and the determination unit is configured to determine whether or not the portable authentication terminal is within the vehicle cabin based on whether or not the information received by the information reception unit is information based on the sound generated by the sound generation unit.