Wulfie, Ardy, Kyrah, Cyrule thank you for your advice.
I changed the setting to solve Z-fighting.
this is effective in Blender, but is not reflected in SL in-world...
I do not make rendering.
I understand the difference between Alpha Masking and Alpha Blending.
When I make it,
show texture on the both sides using "Solidfy" or NOT? or mixes these?
I try such a work.
but my problem has not been fixed...:'(