PURPOSE: To provide a secure PN generating circuit from which the structure of a PN signal and of the PN generating circuit are not analyzed.

CONSTITUTION: The PN generating circuit is provided with noise generating means 1a, 1b, 1c, a replacement section 3 and a control section 7. The noise generating means 1 is constituted of a linear feedback shift register means 11 outputting plural pseudo random noise sequences with a control signal selecting plural sequences and a nonlinear logic 13 fetching plural output signals from the register means 11 and outputting one noise signal. The noise signal from each of the noise generating means 1a, 1b, 1c is given to the replacement section 3, in which various combinations are selected by using a switching signal Sk from the control section 7. The setting of the revision, switching and replacement of the noise generating means 1a, 1b, 1c and the replacement section 3 is revised at any time via the control section 7 and the analysis of the structure of a PN signal and a PN generating circuit is disable.