Hi Yuki,
Please see Adding random questions section in http://docs.moodle.org/19/en/Editing_a_quiz
In reply to Pere O.
Re: How to Add Questions to an Existing Quiz in Moodle 1.9
by HOSAMUDDIN SAHIL -
How to Add Questions to an Existing Quiz in Moodle 2.7
In reply to HOSAMUDDIN SAHIL
Re: How to Add Questions to an Existing Quiz in Moodle 2.7
Please see Building Quiz and remember that if user have already taken your quiz, you cannot then add questions to it.