diff -urN rt_v60/cross.c rt_v61/cross.c
--- rt_v60/cross.c	2018-07-06 03:39:31.000000000 +0900
+++ rt_v61/cross.c	2018-07-13 20:09:46.000000000 +0900
@@ -334,7 +334,7 @@
 	if(kind_idx != KIND_BALL){
 		double *p = ret->l_nv_g.p;
 		double *v = ret->l_nv_g.v;
-		double x[3], z[3];
+		double x[3], y[3], z[3];
 		struct vecs vs;
 	
 		if(is_plane(kind_idx)){
@@ -354,7 +354,11 @@
 		v_sub(x, p, x);
 		v_sub(z, p, z);
 		vecs_by_fix_ref(&vs, 'x', x, 'z', z);
-		v_set( v, vecs_get_by_name(&vs, 'y') );
+		v_set( y, vecs_get_by_name(&vs, 'y') );
+		if(dot(y, v) < 0){
+			v_neg(y, y);
+		}
+		v_set(v, y);
 	}
 
 	if(rev){