mruby forum released mruby 1.0.0 in Feb. 2014. It's been stable to use. However, because mruby is used on small boards or in application programs, it's hard to debug mruby applications. When we develop mruby application, we should write not only Ruby code, but also C code. So we need to debug both codes. To make debugging mruby application easier, we are writing prototype of mruby debugger. Current mruby distribution has no debugger, but there are some useful API sets to implement own debugger. In this talk, we introduce their API and how to use them to write a debugger.