Recently, highly sensitive helium leak detectors with a lower detection limit of under 10-12 Pam3/s have been required in fields which use highly airtight systems. To satisfy this requirement, a typical solution has been to use a secondary electron multiplier as an ion collector or to use a double turbo molecular pump system. But these systems have the problems of unstable sensitivity and higher system costs. To avoid these problems, we developed a highly sensitive magnetic deflection type mass spectrometer by optimizing the ion optics.