Gravity survey was conducted to investigate subsurface structure of the Bupyung caldera. Two profiles of about 23 ㎞ and 21㎞ length were chosen across the caldera and gravity values were measured at 333 points on those profiles. The regional gravity trend of the survey area is mainly attributed to isostasy. The maximum negative gravity anomaly over the caldera appears to be -8.5m gal. Gravity profiles were analyzed and interpreted by using power spectrum analysis, Marquardt-Levenberg inversion and ideal-body inversion methods. The result of 2-D inversion shows the existence of the low density structure of about 8km surface width and 3km depth under the caldera. Since the caldera has three dimensional, 2-D gravity inversion results in relatively large error as compared with 3-D modeling. Fortran subroutine was coded to calculate the gravity anomalies caused by 3-D subsurface bodies. The result of 3-D modeling shows the Bupyeong caldera can be approximated by basin-shaped low density body whose depth reaches to 3㎞. At this depth level we assume that there exist a nearly flat boundary between granite batholith and metamorphic rock out side of the caldera. The negative anomaly of Bupyeong caldera appears to be caused by density contrast between low-density granite inside the caldera and surrounding metamorphic complex. Relationship between the defect mass and the diameter of Bupyeong caldera satisfies general relationship observed in world-wide volcanic calderas.