Hi Yuki,
I had the same problem. If you're converting from text, the answer is actually really simple. Just "underline" the chapter headings by putting a row of dashes underneath and the conversion program will pick it up. like this:
Chapter1
----------
<content>
Chapter 2
-----------
more information and a demo file here:
https://www.mobileread.com/forums/showthread.php?t=32702