The dependence of H- extraction probability on filter magnetic field strength and gas pressure is studied in a small volume-type negative ion source. Spatial distribution of H- density in an ion source and the number of extracted H- ions from a laser path are measured by photodetachment assisted with Langmuir probe and by photodetachment assisted with Faraday cup, respectively. In our experimental conditions, the filter magnetic field affects the H- extraction probability in the extraction region at the low gas pressure. Based on these experimental conditions, H- gyro orbit is simulated considering the collisions between H- and other particles of H-2, H, and H+. The simulated results explain well the H- transport mechanism in the extraction region. (c) 2005 Elsevier B.V. All rights reserved.