1. A crystal unit, comprising:an AT-cut crystal element, having a planar shape which is approximately a rectangular shape; and
a container, secured to the AT-cut crystal element at a side of a first side as one side of the rectangular shape by a securing member,
wherein the AT-cut crystal element includes:
a first inclined portion, inclined such that the AT-cut crystal element decreases in thickness from a proximity of the first side to the first side;
second inclined portions, disposed on respective both ends of the first side, the second inclined portions being formed integrally with the first inclined portion, the second inclined portions being inclined gentler than the first inclined portion; and
a first secured portion and a second secured portion, formed integrally with the second inclined portions, and the first secured portion and the second secured portion each projecting out in a projecting direction which is from the first side to an outside the AT-cut crystal element to be used for securing with the securing member.