Hi @HaeryangKim and welcome,
I'd probably go for either a SICK's TIM or a Hokuyo which have nice ROS drivers.
We provide autopilot interfaces through the mavros ROS package. Have a look at its interfaces here.
I'd probably use a range finder for that. [Here](http://wiki.ros.org/Sensors#A1D_range_finders you have a list of 1D range finders with ROS support that will do the job.
Alternatively, these are the supported range finders within APM. Theoretically, you'll get measurements integrated in the autopilot but note that we haven't tested extensively the improvements that the range finder add on top of the default (barometer-based) altitude estimation.