An ultrasonic system comprising: an ultrasonic data obtaining unit obtaining volume data that corresponds to a 3-dimensional image of an object, the object comprising a target object; a user input unit receiving input information; and a processor, generating the 3D image by using the volume data, detecting 3D geometry information that corresponds to a 3D marker in the volume data based on the input information, and setting the 3D marker on the 3D image based on the 3D geometry information that is detected, wherein the input information comprises: first input information for setting a point on the 3D image, wherein the point corresponds to the 3D marker; second input information for selecting the target object; and third input information for setting a reference value for determining a location of the 3D marker in the 3D image in a depth direction, wherein the processor sets a reference value that corresponds to the target object, based on the second and the third input information; sets a virtual observation plane that is formed of a plurality of pixels, based on the volume data; detects a pixel which is on the virtual observation plane corresponding to the point set by the first input information; projects a virtual ray from the pixel that is detected to the volume data; obtains a sampling point and a sampling value of the sampling point by sampling the virtual ray at a predetermined sampling interval; detects, according to the sampling value, a voxel that corresponds to the reference value from the volume data; and determines the 3D geometry information of the point by using 3D geometry information of the voxel that is detected. an ultrasonic data obtaining unit obtaining volume data that corresponds to a 3-dimensional image of an object, the object comprising a target object; a user input unit receiving input information; and a processor, generating the 3D image by using the volume data, detecting 3D geometry information that corresponds to a 3D marker in the volume data based on the input information, and setting the 3D marker on the 3D image based on the 3D geometry information that is detected, wherein the input information comprises: first input information for setting a point on the 3D image, wherein the point corresponds to the 3D marker; second input information for selecting the target object; and third input information for setting a reference value for determining a location of the 3D marker in the 3D image in a depth direction, wherein the processor sets a reference value that corresponds to the target object, based on the second and the third input information; sets a virtual observation plane that is formed of a plurality of pixels, based on the volume data; detects a pixel which is on the virtual observation plane corresponding to the point set by the first input information; projects a virtual ray from the pixel that is detected to the volume data; obtains a sampling point and a sampling value of the sampling point by sampling the virtual ray at a predetermined sampling interval; detects, according to the sampling value, a voxel that corresponds to the reference value from the volume data; and determines the 3D geometry information of the point by using 3D geometry information of the voxel that is detected. first input information for setting a point on the 3D image, wherein the point corresponds to the 3D marker; second input information for selecting the target object; and third input information for setting a reference value for determining a location of the 3D marker in the 3D image in a depth direction, wherein the processor sets a reference value that corresponds to the target object, based on the second and the third input information; sets a virtual observation plane that is formed of a plurality of pixels, based on the volume data; detects a pixel which is on the virtual observation plane corresponding to the point set by the first input information; projects a virtual ray from the pixel that is detected to the volume data; obtains a sampling point and a sampling value of the sampling point by sampling the virtual ray at a predetermined sampling interval; detects, according to the sampling value, a voxel that corresponds to the reference value from the volume data; and determines the 3D geometry information of the point by using 3D geometry information of the voxel that is detected.