Finally I found the problem.
My z-axis conduit was not orthogonal to x-axis conduit, it was slightly tilted to left.
So it was shifted to left as height increased.
After fixing the tilting, shifting is disappeared and 15Cm height printing is successful with 50mm/s speed.
There is no need to decrease z-axis height.. (my build area is still 400mmX400mmX250mm)
One more thing I changed is the leg (https://www.thingiverse.com/thing:902483).
It has only 2 screw holes so it can stand the shaking with one direction, additional 2 holes are added to the opposite side to fix the legs firmly.
(I replaced the legs with stainless ones at last like the attached picture)