(Blender3D) set origin to center of cube that belong bigger mesh model

1. move cursor to center of cube

1-1. In Edit mode, select two faces facing each other.

1-2. 'Shift S' hotkey, use snap.  'cursor to selected'

2. move origin to cursor

 2-1. change to Object mode, right mouse click or 'Object' menu below

 2-2. set origin > origin to 3D cursor

3. Done!