  This paper has been withdrawn by the author. The author found that the main
results here were already obtained by K. Taniyama and A. Yasuhara `On
$C$-distance of knots. Kobe J. Math. 11 (1994), no. 1, 117--127. MR1309997
(95j:57010)'. He would like to thank M. Ozawa for the information.
