To provide a highly reliable data transfer circuit, which has a fault detecting function suppressing an increase in circuit scale although the constitution of a principal and a sub circuit is adopted, and a fault detecting method.

While the principal circuit transfers input data, the sub circuit compresses the data path width of the input data and then transfers the data, and a comparator is provided which detects a fault of the principal circuit by comparing compressed data generated by compressing the data path width of the output data of the principal circuit with the output data of the sub circuit.