The crystal oscillator has a configuration where circuit elements including a crystal unit are arranged on a mounting board comprising external terminals, opening end faces of a concave metal cover are made to touch the surface of the mounting board, clearances from the opening end faces are comprised in the central regions at both ends in the width direction of the metal cover, protruding parts, which extend from the opening end faces and have a protrusion on an inner face, are comprised at both ends in the longitudinal direction of the metal cover, and each of the protruding parts is elastically inserted in a groove provided on both side faces in the longitudinal direction of the mounting board 1 and bonded by solder, wherein the tip side of each protrusion is thrust and bites into a metal film provided in the groove.