I took it to second transmission service place yesterday and got same answer. They say pipe is in good shape, this mettal fitting between pipe and body is leaking because of rust probably. They called Isuzu dealers and found that this fitting is not manufactured any more.
I think to try take it apart in my garage tonight and see what I can do. Worsed thing could happen - this fitting will brake because of rust and i'll have part of it in my transmission body :P

Thank you all for responce!