An apparatus for monitoring the surrounding environment of a vehicle, comprising: a sensor unit comprising a plurality of detection sensors (110, 120, 1230, 140) configured to detect an object outside the vehicle according to frames with a predefined period; and a control unit configured to extract a stationary object among outside objects detected through the sensor unit by using behavior information of the vehicle, map the extracted stationary object to a preset grid map, add occupancy information to each of grids constituting the grid map depending on whether the stationary object is mapped to the grid map, calculate an occupancy probability parameter indicating the probability that the stationary object will be located at each of the grids, from the occupancy information added to the grids within the grid map in a plurality of frames to be monitored, and monitor the surrounding environment of the vehicle on the basis of the calculated occupancy probability parameter, wherein the grid map has a threshold value defined for each of the grids on the basis of a mathematical model according to the intensity of a received signal inputted to the sensor unit, the threshold value being used to decide whether the stationary object occupies each of the grids within the grid map, characterized in that the threshold value has different values for an independent area, a single-overlap area, and a multi-overlap area within the grid map, wherein the independent area is an area within the grid map, sensed by a first detection sensor in a K th frame, where K is a natural number, the single-overlap area is an area within the grid map, where the area sensed by the first detection sensor and the independent area overlap each other in a (K+1) th frame distinguished from the K th frame, and the multi-overlap area is an area within the grid map, where an area sensed by a second detection sensor adjacent to the first detection sensor and the single-overlap area overlap each other in the K th frame or the (K+1) th frame. a sensor unit comprising a plurality of detection sensors (110, 120, 1230, 140) configured to detect an object outside the vehicle according to frames with a predefined period; and a control unit configured to extract a stationary object among outside objects detected through the sensor unit by using behavior information of the vehicle, map the extracted stationary object to a preset grid map, add occupancy information to each of grids constituting the grid map depending on whether the stationary object is mapped to the grid map, calculate an occupancy probability parameter indicating the probability that the stationary object will be located at each of the grids, from the occupancy information added to the grids within the grid map in a plurality of frames to be monitored, and monitor the surrounding environment of the vehicle on the basis of the calculated occupancy probability parameter, wherein the grid map has a threshold value defined for each of the grids on the basis of a mathematical model according to the intensity of a received signal inputted to the sensor unit, the threshold value being used to decide whether the stationary object occupies each of the grids within the grid map, characterized in that the threshold value has different values for an independent area, a single-overlap area, and a multi-overlap area within the grid map, wherein the independent area is an area within the grid map, sensed by a first detection sensor in a K th frame, where K is a natural number, the single-overlap area is an area within the grid map, where the area sensed by the first detection sensor and the independent area overlap each other in a (K+1) th frame distinguished from the K th frame, and the multi-overlap area is an area within the grid map, where an area sensed by a second detection sensor adjacent to the first detection sensor and the single-overlap area overlap each other in the K th frame or the (K+1) th frame.