To improve test pattern efficiency by extracting an ATM cell data string from frame data for testing for which the data length of an area for storing ATM cell data is reduced and testing the same overhead communication function as a real mode.

At the time of a test mode, the data length of an SPE area for storing the ATM cell data is reduced to 51 bytes × 9 rows to 261 bytes ×9 rows in a normal transmission frame. An overhead area is kept as 9 bytes ×9 rows as it is. A frame header position detection circuit 1 detects the position of a frame header and outputs overhead timing signals based on overhead data. In this case, while it is 2430 clock cycles in the real mode, it becomes 540 clock cycles in the test mode. An SPE area data extraction circuit 3 detects the leading byte position of the SPE area based on the overhead timing signals and extracts SPE area data.