A method of registering medical images, the method comprising: setting, as a reference axis and a coordinate axis of a coordinate system used by a detection apparatus, an axis of a probe, as a result of user command indicating that the axis of the probe has been disposed to be parallel to an axis of an object; using the probe and generating a first medical image, the first medical image being of a first modality; and extracting a first sub-object from the first medical image that is obtained by using the probe; and extracting a second sub-object from a second medical image, the second medical image being of a second modality that is different from the first modality; and registering the first medical image and the second medical image by aligning the first sub-object and the second sub-object; wherein the axis of the object is parallel to a coordinate axis of the second medical image; wherein the registering comprises calculating a conversion relationship between a coordinate system of the first medical image and a coordinate system of the second medical image based on aligning the first sub-object and the second sub-object; wherein the calculating of the conversion relationship comprises rotating a coordinate axis of the first medical image to be parallel to the reference axis and moving the rotated coordinate axis of the first medical image to align the first sub-object and the second sub-object, wherein moving the rotated coordinate axis includes at least one of a rotation and a linear movement; and characterised in that the aligning comprises matching a geometric arrangement between the first sub-object and the second sub-object with a geometric arrangement that is previously stored, when the first sub-object and the second sub-object represent different physical objects. setting, as a reference axis and a coordinate axis of a coordinate system used by a detection apparatus, an axis of a probe, as a result of user command indicating that the axis of the probe has been disposed to be parallel to an axis of an object; using the probe and generating a first medical image, the first medical image being of a first modality; and extracting a first sub-object from the first medical image that is obtained by using the probe; and extracting a second sub-object from a second medical image, the second medical image being of a second modality that is different from the first modality; and registering the first medical image and the second medical image by aligning the first sub-object and the second sub-object; wherein the axis of the object is parallel to a coordinate axis of the second medical image; wherein the registering comprises calculating a conversion relationship between a coordinate system of the first medical image and a coordinate system of the second medical image based on aligning the first sub-object and the second sub-object; wherein the calculating of the conversion relationship comprises rotating a coordinate axis of the first medical image to be parallel to the reference axis and moving the rotated coordinate axis of the first medical image to align the first sub-object and the second sub-object, wherein moving the rotated coordinate axis includes at least one of a rotation and a linear movement; and characterised in that the aligning comprises matching a geometric arrangement between the first sub-object and the second sub-object with a geometric arrangement that is previously stored, when the first sub-object and the second sub-object represent different physical objects.