After playing nanashogi more than tree years,
I have realized that unfortunately rules of the game
contain some weakness:
"Black has too much power to win the game".

Developing of the rules has been necessary.
In fact the rules are still the same,
but two characters, Rook and Swallow's Wings have
new limitted way to move:
Rook moves just one step to the right, left, forward or back,
and
Swallow's wing moves just one step to the right or left.