To reduce testing cost by dispensing with an external testing function by deciding a fault by comparing an average value calculated from a level of a test signal inputted from a sound input line with an expected value.

A folded loop of test signal is formed in an interface device, by connecting a test signal generator 18 with a D/A converter 12 with a switch 19 and connecting output and input lines 14 and 15 with a switch 20 under an instruction of a host processor 11 at self-diagnosis. A test signal digitally converted by an A/D converter 17 is sampled for n times by a signal level detector 21, and average sampling value is calculated. The average value is determined by whether it exists within a range of expectation values by an acceptance deciding device 22, if it exists within the range, a fact that it is normal is notified to the host processor 11 and when it exists out of the range, a fact of a fault is notified to the host processor 11. Thus, since operation test is performed without external connection of equipment for test, required cost for construction test environment is reduced, and on-line fault diagnosis of product is enabled.