A crystal oscillator for surface mounting comprising: a case main body including concave portions on both principal surfaces; a crystal element hermetically encapsulated in one concave portion; an IC chip housed in the other concave portion; mounting terminals provided on four corner portions of an opening end face of the other concave portion; and a protrusion serving as identifying marks along a direction of sides of the case main body and provided in some of the mounting terminals. The mounting terminals, to which the protrusions are provided, are provided on both corner portions on one end side of the case main body. The protrusions extend in a same direction from the mounting terminals at an inner or outer circumference side of the opening end face. The mounting terminals including the protrusions are symmetric with respect to a center line between both corner portions on the one end side.