Boroxine cages are nanometer-sized covalent cage-like molecules that utilize beroxine formation reactions. Such molecular-sized hollow structures can contain other molecules. Encapsulated molecules sometimes change their properties significantly, and various applications using them are being investigated. One of the methods for confirming the synthesis of boroxine cages is mass spectrometry, and MALDI-TOFMS is suitable because it can ionize a wide range of compounds mainly as single-charge ions. The JMS-S3000 SpiralTOF™ achieves a long flight distance of 17 m due to its unique spiral ion trajectory, and can measure low-molecular-weight compounds ionized by the MALDI method with high mass resolution and high mass accuracy. In this report, we report the accurate mass measurement of the boroxine caged 12-mer.