The man who set the Pennsylvania Governor's Mansion ablaze in April pleaded guilty today to arson and other charges ... and new video released shows the crime -- and inferno -- in progress.

Watch the video ... you can see Cody Balmer break a window of Pennsylvania Governor Josh Shapiro 's home from the outside, light a Molotov cocktail, and toss it in, starting a blaze.

Balmer, a mechanic, enters the residence through another broken window ... you can see him roaming around inside the home trying to enter various rooms by kicking the doors in. When that fails, he just starts throwing down more explosives in what appears to be the dining room ... before casually exiting through a front door and strolling away across the lawn.

Shapiro and his family were sleeping inside the house at t

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