Frank Beard -- the longtime drummer of the rock band ZZ Top -- is dealing with serious health issues ... and it's so bad it's affecting their tour ... TMZ has learned.

The health update comes just days after ZZ Top cancelled their August 5 show at the Hollywood Bowl.

Sources tell TMZ ... Frank's health was the reason behind the cancellation.

At the time, frontman Billy Gibbons said on social media ... "Insurmountable obstacles require canceling our Hollywood Bowl performance."

He continued ... "Friends, fans and followers of ZZ Top are the best in the business and the band is counting on everyone to bear with us for this brief interruption."

Drummer John Douglas has subbed in for Frank in the past ... like last year, when Frank briefly stepped away from the band "to attend to a health issue" ... but in Frank's recent absence, it's been Michael Monahan.

Frank's been with ZZ Top since 1969.

ZZ Top performed without Frank on Saturday night in Saratoga, CA ... and their next show is scheduled for Tuesday in Redmond, WA.