To easily test entire connection and at the same time relieve the load of hardware and the load for creating test data when a register used inside a user I/O block need neither read nor write data to or from an MPU.

In a microprocessor system, a scan control part 14 serially transfers data from registers 5 and 6 to a scan register 13 exclusively for testing by scanning when an MPU 2 instructs the registers 5 and 6 exclusively to output data and serially transfers data from the scan register 13 exclusively for testing to the registers 7 and 8 by scanning when the MPU 2 instructs the registers 7 and 8 exclusively for reading to write data.