diff -urN rt_v56/cross.c rt_v57/cross.c
--- rt_v56/cross.c	2018-06-29 09:16:30.000000000 +0900
+++ rt_v57/cross.c	2018-06-29 09:20:42.000000000 +0900
@@ -410,12 +410,17 @@
 		r->t = r->ts[0];
 		line_on_line_p( &r->l, r->t, r->p );
 		cross_get_nv( d->kind, &d->l2g, l_g, 0, r );
+		if(r->ang_nv_eyev == 0){
+			r->ts_n = 0;
+			cross_op_ins(l_g, prev_idx, data, a, r);
+			return;
+		}
 		r->flags = 0;
 		if( prev_idx == r->idx ){
 			r->flags |= CROSS_FLAG_PREV;
 		}
 		area_ins( a, r->t, r, sizeof(*r) );
-		if(r->ang_nv_eyev >= 0){
+		if(r->ang_nv_eyev > 0){
 			area_not(a);
 		}
 	}else{ /* n == 2 */