OK- the bike looks like a collection of random parts. (For instance, the headset locknut looks really out of place on that fork.) I would guess that someone's replaced a standard-head bolt with a hex-head (aka Allen-head) type. If it holds without slipping, don't mess with it.

OTOH- are you trying to remove the stem? If you've unscrewed the bolt a couple turns, give it a tap with a hammer. There's a wedge at the bottom of the stem that won't move unless you break it loose.