point of RaycastHit is changing, when ray is not.

Hello. I wanna make 3d gameobject to follow touchpoint. But the Vector3 axis of RaycastHit is changing, while I'm not moving my cursor. Below is my code and debug log. is raycastHit axis supposed to be changing like this? Y position of debug log is variating, but I didn't move the cursor.

Y axis is increasing by 0.5, and has ended up at 29.5. I think it's because Y axis of mainCamera is 30. mainCamera is looking at below of itself.