According to Frank's death certificate, obtained by TMZ, the immediate cause of death is "late sequela of cerebral infarction" ... medical jargon for a stroke.

The document says Frank's fatal stroke was a result of cerebral vascular disease ... a condition that affects blood flow and blood vessels in the brain.

Frank also suffered from aortic stenosis -- a chronic condition that makes it difficult for blood to flow from the heart to the rest of the body -- and COPD ... according to the certificate, with both conditions playing a role in his death.

As we reported ... Frank died Sept. 30 in a hospice facility in the Midwest, surrounded by friends, including his longtime "American Pickers" costar Mike Wolfe.

Frank's rep previously told us he died from the effects of a stroke he had several years ago ... and that's confirmed in the death certificate.

The documents also say the official time of death was 8:50 PM, the manner of death was natural and his body has since been cremated.

Frank was 60.