A chemical ionization mass spectrometer (CIMS) and an automated air sampling/background signal measurement system designed for fast, reliable, and continuous ground-based measurement of gas-phase nitric acid (HNO3) were developed and characterized in a remote marine boundary layer site, Rishiri Island Observatory in Japan, under various meteorological conditions. HNO3 transmission efficiency of air sampling line, interference of NO and NO2 by corona discharge ion source, time response, detection sensitivity, and detection limit of the system were determined under the ambient condition. Detection limit of the system, defined as 3 times the standard deviation of background signal, varied depending on the atmospheric HNO3 concentration, 3--5 parts per trillion by volume (pptv) for the clean condition (HNO3 < 100 pptv) and 15--20 pptv for the polluted condition (HNO3 1--2 ppbv) with 2-s integration time. The determining factor of HNO3 transmission efficiency (HNO3 loss) and critical points for reliable and fast measurement of gas-phase HNO3 in the marine boundary layer were identified on the basis of field and laboratory tests of the CIMS system.