Jacques Schaap was charged with three counts of making indecent images of youngsters when he appeared at Preston Magistrates Court.
The 51-year-old admitted all three charges.
He was charged with making 231 category A images, 248 category B images and 438,518 category C images.
The offences took place during a 19-year spell of offending between September 2002 and April 2021.
He was remanded on unconditional bail but must report to Chorley police station on a regular basis.
Schaap will be sentenced at Preston Crown Court on June 27.