An object of the invention is to provide a mass spectrometry device and an ion detection method therefor in which an ion amount can be detected with high accuracy.In order to achieve the object described above, a mass spectrometry device is provided which performs a channel scan measurement by changing a voltage to be applied to a mass separation unit to selectively extract a desired ion. The mass spectrometry device includes: an ion detection unit which detects an ion separated by the mass separation unit and outputs an electric signal; an ion amount measuring unit which measures an ion amount from the output of the ion detection unit; and an ion amount correction unit which corrects a detection amount of ion from an output of the ion amount measuring unit. In a process of a channel scan, the ion amount correction unit performs correction of a detection amount of ion detected in a present channel based on a detection amount of ion in a previous channel.