Ultra Wide-Band (UWB) pulse radar provides promise for surveillance systems via its high-range resolution. To realize a low-cost and high-quality indoor security system, we propose a UWB radar imaging system using indoor multipath echoes for targets in shadow regions. A multipath wave can be used as an approximation of an imaginary echo from a mirror image antenna to the target, ignoring phase rotation and attenuation. Conventional studies have only dealt with locating point-like targets, not estimating their shape. We apply interferometry using these mirror image antennas to estimate target shapes. If this method alone is applied, many false image points are estimated because it is difficult to determine uniquely the corresponding mirror image antenna to each echo. We propose an effective false-image reduction algorithm to obtain a clear image. Numerical simulations show that most of the false image points are removed and the target shape is accurately estimated.