Ben Affleck Pulls Out Carton of Cigarettes While Picking Up Kids

Ben Affleck's diet appears to consist of Dunkin' Donuts and cigarettes -- and yet ... he's in tip-top shape, with a dad bod most of us would kill to have.

The actor was picking up his kids in L.A. Monday -- specifically, his 14-year-old daughter Seraphina and 11-year-old son Samuel -- and he was in complete Papa Bear mode ... of the big-rig variety, though, it seems.

Check it out ... while waiting with Seraphina in the parking lot, he started rummaging through a backpack and pulled out a giant carton of Marlboro cigs, which was already torn open.

Once Sam showed up and the 3 of them started heading out ... Ben had a Dunkin' coffee cup with him, and it looked pretty empty. We know Ben's a fan of the coffee/donut brand ... so it's not all that crazy to see him with one of these.

What is interesting ... the fact Ben tends to snag fast food pretty often -- including a McDonald's run he made last week, where he had a bag of grub and a tasty drink to boot.

Lop on the fact he's a regular smoker ... and it's a little surprising Ben's able to stay in as good of shape as he does. We know the guy's shredded thanks to a snap his wife, J Lo, shared on Father's Day -- so he must hit the gym regularly to keep things balanced.