Abstract: A security apparatus for motor vehicles is disclosed, and is featured in integrating all the components into one body as an all-in-one form and in filling resin into the housing of the security apparatus so as to completely cover an electric circuit unit and the interior surface of a siren. The present invention is alternately featured in providing a metal cover covering the externally-exposed surface of the siren for protecting the siren; providing fastening elements each of which has a head hole of special pattern, thereby preventing the security apparatus from being unlawfully dismantled; and providing several detecting modes and power-saving modes, so as to effectively and reliably provide the antitheft function for motor vehicles.