The objective of the presently disclosed subject matter is to provide a mass spectrometer that improves the ionic permeability ratio at the entrance to an ion transport part or the entrance to a mass spectrometry part, and to acquire a high-sensitivity mass spectrum. To reduce the electric-field distortion that is caused by ion loss, electrodes are arranged so that the radius of a circle inscribed within the electrodes of the ion transport part is larger than the radius of a circle inscribed within the electrodes of the mass spectrometry part. The entrance to the electrodes of the mass spectrometry part also can have a tapered, inclined, folded-over, or rounded configuration. Together, these reduce the sharply fluctuating (peak-shaped) distribution of electric potential generated near the entrance to the ion transport part and the entrance to the mass spectrometry part.