LeBlanc ventured out Thursday afternoon in Sherman Oaks, CA ... driving in his Mercedes-Benz SUV. It's the first time we've seen him in public since Matthew died over last Saturday.

Matt's wearing dark sunglasses and a baseball cap, and looks pretty sad ... a reflection of the emotions he and his former cast mates expressed earlier this week.

Remember, ML and the rest of the tightknit "Friends" stars have said they are "utterly devastated," and are still processing the fact they've lost Matthew.

In their statement, Jennifer Aniston, Courteney Cox, Lisa Kudrow, David Schwimmer and Matt said ... "We were more than just cast mates. We are a family. There is so much to say, but right now we're going to take a moment to grieve and process this unfathomable loss."

They added, "For now, our thoughts and our love are with Matty's family, his friends, and everyone who loved him around the world."

TMZ broke the story ... Matthew was found unresponsive in his hot tub, after he'd played 2 hours of pickleball on Saturday morning. Sources say there were no illicit drugs found at his home, though there were several Rx medications including anti-depressants and anxiety meds.

As we first told you, Matthew went out to lunch with model and entertainment reporter Athenna Crosby the day before he died, and she says he was happy and vibrant.

Matthew's full toxicology report could take up to 6 months to complete, though initial testing revealed he didn't have meth or fentanyl in his system.