Thank you for your answer. I understood your suggestion correctly. Now, let me show you my argument.
In the original coordinate, we have the following.
$$ z \sim \frac{1}{2} \vec {x}^{T} M\vec {x} (1)$$
(Top of page 84)
In the rotated coordinate, we have the following.
$$
z \sim \frac{1}{2}...