PURPOSE: To provide a test device and a method which test high speed data transfer speedily and accurately and give proper diagnosis information at the same time.

CONSTITUTION: An undersampling digital test circuit 20 includes a means which guides data transfer, a data collection array 22, and a frequency divider circuit 18. The frequency divider circuit 18 supplies an enable signal to the data collection array 22 which undersamples data which advances along a means which guides data transfer at high data transfer speed to test the completeness of high data transfer speed transfer effectively without bringing about disadvantages of a test method based on the conventional technique.