Lightmap baking cause noisy result

Hello.

I'm building an indoor dungeon styled game, with a lot of light sources, but I set most to baked. Then after baking, this is the result:






I have no idea why it's noisy like this, instead of a smooth result.
I tried both progressive and enlighten setting, with various different resolution, but the result still more or less the same.

Anyone know why this happens, and how I can solve this?

Thank you.

P/S: I'm using Unity 2018.2.12f1 64 bit.