An integrated circuit device includes a circuit device main body which is configured to execute a predetermined processing function, a communication control circuit which is configured to perform data communication with an external control device and an operation mode determination unit which is configured to selectively determine a normal mode for executing the processing function or a debug mode for setting an execution condition of the processing function as an operation mode of the circuit device main body. The operation mode determination circuit is configured to operate in accordance with an internal clock and to generate an operation mode output value for determining the operation mode of the circuit device main body according to a logical state of a particular one communication signal which is data-communicated with the external control device after a reset operation performed by a reset circuit is released.