To provide a turbo decoder improved in error correction ability and throughput.

Concerning a turbo decoder 1 for performing turbo decoding by inputting turbo-encoded data by the unit of a block, the S/N (signal-to-noise ratio) of a signal under processing is estimated by performing the -mean square of soft output data L2 (u*) from a soft output decoder 12 in a -mean square circuit 19, an IER corresponding to that S/N is outputted from a look-up table 23, and the levels of input data Ys, Yp1 and Yp2 are controlled by input level control parts 20, 21 and 22.