A semiconductor integrated circuit system includes a first power line which supplies a first source power voltage, and a second power line which supplies a second source power voltage. A first edge detecting unit outputs a first edge detection signal when a rising edge of the first source power voltage is detected. A second edge detecting unit outputs a second edge detection signal when a rising edge of the second source power voltage is detected. An output unit is connected to the first power line, and outputs data to a data terminal in a data output cycle and sets the data terminal in a high-impedance state in response to the first edge detection signal. An output control unit is connected to the second power line, and controls the output unit in accordance with a read-data signal in the data output cycle, and controls the output unit in response to the second edge detection signal, so that the data terminal is set in the high-impedance state by the output unit.