I meaured the laser intensity noise with free-running and estimated the requirement for ISS.
The RIN with free-run laser was the order of 10^-6.

Assuming DC lock of HOMs and common mode rejection ratio as 1/10, the requirement for RIN is estimated as 3*10^-8.

Therefore, about 100-times stabilization is necessary in order to achieve the requirement.
I will design the intensity stabilization servo which satisfies this requirement.