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
by Mary Cooch -
Please see Building Quiz and remember that if user have already taken your quiz, you cannot then add questions to it.