Abstract: A transmission test apparatus having a system-clock redundancy system for testing a transmission path is proposed. In the transmission test apparatus, a clock switching unit selects a proper one from a plurality of system clocks in response to a clock selection signal and supplies a selected system clock. A test-signal check unit performs a synchronization operation in which a first test pattern generated based on the selected system clock is synchronized with a test signal coming through the transmission path and compares the first test pattern with the test signal. A synchronization detection unit detects the synchronization between the first test pattern and the test signal by examining the comparison result of the test-signal check unit to produce a synchronization detection signal. An error counter performs a counting operation in which a number of unidentified bits is counted as error bits after receiving the synchronization detection signal.