By using function one direction S-rough sets (function one direction singular rough sets), the concept of rough law is proposed, where rough law is the law pair {w(x)_-, w(x)^-} composed by w(x)_- and w(x)^-. If the attribute set {¿_-, ¿^-} of function one direction S-rough sets changes, then rough law {w(x)_-, w(x)^-} will change, too. By employing ellipse curves, the concept of rough law security is presented, the security theorem of rough law is proposed and the applications are given. Rough law security, which is generated by the intersection and penetration between function one direction S-rough sets and ellipse curves, is a new applied research direction of function S-rough sets.