A communication system comprising: a vehicle; and a communication device communicating with said vehicle, said communication device including a signal generating unit coupled to ground and being adapted to generate and output a signal having a predetermined potential, a first terminal for connecting said communication device with said vehicle and performing a communication with said vehicle, a first resistance circuit connected between an output of said signal generating unit and said first terminal and having a resistance value varying according to a first input signal, said first input signal being set by said communication device in accordance with an operation mode of said vehicle, said operation mode including a charging mode and a power generation mode; said vehicle including a second terminal connectable to said first terminal, and a second resistance circuit connected between a node coupled to the ground potential and said second terminal, said vehicle being provided with a first signal extraction node on a path connecting said second terminal and said second resistance circuit to each other; and said communication circuit further including a voltage sensor for detecting a voltage at a node provided on a path connecting said first terminal and said first resistance circuit for reading a value obtained by said second resistance circuit and said first resistance circuit, wherein a change in the value indicates that the preparation corresponding to the operation mode has been completed in the vehicle. a vehicle; and a communication device communicating with said vehicle, said communication device including a signal generating unit coupled to ground and being adapted to generate and output a signal having a predetermined potential, a first terminal for connecting said communication device with said vehicle and performing a communication with said vehicle, a first resistance circuit connected between an output of said signal generating unit and said first terminal and having a resistance value varying according to a first input signal, said first input signal being set by said communication device in accordance with an operation mode of said vehicle, said operation mode including a charging mode and a power generation mode; a signal generating unit coupled to ground and being adapted to generate and output a signal having a predetermined potential, a first terminal for connecting said communication device with said vehicle and performing a communication with said vehicle, a first resistance circuit connected between an output of said signal generating unit and said first terminal and having a resistance value varying according to a first input signal, said first input signal being set by said communication device in accordance with an operation mode of said vehicle, said operation mode including a charging mode and a power generation mode; said vehicle including a second terminal connectable to said first terminal, and a second resistance circuit connected between a node coupled to the ground potential and said second terminal, a second terminal connectable to said first terminal, and a second resistance circuit connected between a node coupled to the ground potential and said second terminal, said vehicle being provided with a first signal extraction node on a path connecting said second terminal and said second resistance circuit to each other; and said communication circuit further including a voltage sensor for detecting a voltage at a node provided on a path connecting said first terminal and said first resistance circuit for reading a value obtained by said second resistance circuit and said first resistance circuit, wherein a change in the value indicates that the preparation corresponding to the operation mode has been completed in the vehicle.