Hi Ikumi

I spent some time digging into the internals of preview-latex, even though I do not speak elisp wery well...

The crucial part happens inside preview-gs-transact, which gets called each time GS has written something to the standard output (usually  GS>  or  GS<1>)

As I wrote in a previous message, adding some debug prints to the beginning of preview-gs-transact shows that you need to count the number of GS<1> prompts in order to guess when a new file is ready.
Current code just counts all prompts, skipping one, so with two GS> prompts at the beginning it is off by one.

The attached patch, based on that observation, solves the problem entirely.

Again, as I said in all my previous messages, I DO NOT UNDERSTAND ghostscript, I am just making empiric observations regarding the correlation between what GS writes to stdout and what seems to happen.
These observations seem to be consistent regardless of the color compatibility option you choose or the number of files you feed into GS at a time (but only tested with 9.50)

Cheers,
Itaï