How is the PhDs' (in Logics in Math) employment rate? I am an undergraduate student who is majored in Math now. I plan to continue my phD degree in Logics in Math. My ultimate job plan is to be professor (I know instructor first). So I really want to learn the employment of PhDs in this major.

Thank you for your time.
For mathematics in the US, the American Mathematical Society collects a great deal of data (about employment among many other things) in their Annual Survey of the Mathematical Sciences.  

With respect to your specific question, unfortunately they lump logic together with discrete mathematics, combinatorics, and computer science; presumably logic alone would have too few data points to be very informative.  (The inclusion of computer science here doesn't mean the AMS thinks that computer science is one small subfield of mathematics.  That refers, I think, only to people working on theoretical computer science who received their degrees in departments of mathematics.)

This pdf has the table that most directly addresses your question, for 2014-15 doctoral recipients.