An inner container including a tube body is provided in an outer container. With this structure, the inside of the outer container is separated into a primary airtight chamber and a sub airtight chamber (upper airtight chamber and lower airtight chamber). A detection circuit is placed in the primary airtight chamber. The detection circuit is a tuning and matching circuit having a transmission and reception coil serving as an NMR detection element and a variable capacitor. An additional member including an additional element such as a capacitor is inserted into the upper airtight chamber, and the additional member is set on a top terminal mounting member and a bottom terminal mounting member. With this structure, the additional element is electrically connected to the detection circuit and a characteristic of the detection circuit is changed.