There are provided a metal product having an internal space formed therein, allowing for improvements in the flow of a coolant in the internal space, such as a cooling channel and an increase in cooling efficiency, and a method of manufacturing thereof. The metal product includes a body part having a first space formed therein; a space formation member having a second space formed therein, mounted on the body part to be communicated with the first space; and a finishing part forming an exterior by covering the space formation member in a state in which the space formation member is mounted on the body part.