To examine the effect of change of subsurface flow system on the surface SP (self potential), SP measurements were carried out before and after the pressure buildup and drawdown during the periodic inspection at Nigorikawa area. Relation between the SP distribution and the observed data was also examined by 2-D numerical simulation. Tendency was found that the SP increased gradually with the production near the production well, decreased during the pressure buildup, and increased again during the drawdown. There were some points having the reverse tendency in the surrounding area. Behavior during the pressure buildup and drawdown was not clear. The resistivity near the ground surface was low ranging between 2 and 5 ohm/m within the Nigorikawa basin. The variation of SP was not so large when compared with the measuring error. The SP profiles on the secondary section passing in the center of caldera at the production stop and at one week after the production start were well corresponded with the profiles under natural conditions which were reproduces using the 2-D model. It was considered that the SP profile before the production stop was affected by the production. 12 refs., 6 figs., 1 tab.