An object of the present invention is to provide a technique to improve a manufacturing yield of a gas cell included in a magnetic field measuring apparatus. A gas cell GC according to an embodiment is characterized in that, for example, a cavity CAV includes an opening OP1 and an opening OP2, a plane size of the opening OP1 becomes larger than a plane size of the opening OP2 in the openings OP1 and OP2 included in the cavity CAV. Especially, according to the embodiment, the opening OP2 is included in the opening OP1 in plan view. Consequently, a width of the opening OP1 coming into contact with an upper surface of the sealing substrate 1S becomes larger than a width of the opening OP2 coming into contact with a lower surface of the sealing substrate 2S.