Ex-Korn drummer David Silveria insists he was NOT under the influence at the time of his DUI arrest earlier this year -- pleading "not guilty" earlier this week in an Orange County courtroom.
TMZ broke the story ... Silveria was busted for DUI in Huntington Beach back in March after rear-ending another car.
The man insisted he hadn't a drop to drink -- but told us he did take a sleeping pill the night before, the effects of which may have lasted through the morning ... making him too drowsy to drive safely.
Silveria called it an "honest mistake" -- and has now decided to fight the charges.
A pretrial date has been set for September.