Hello Hiro,
In-camera double exposure is a process in which you take one picture on top of other.
It is possible to do it with digital camera, as well!
Not all digital cameras have this capability. Current models that do include the Canon EOS 5D Mark III, 1D X, and 70D; most Nikon DSLRs; Fujifilm’s X-Pro1 and X100s; and the Olympus OM-D E-M5, among others. With some, combining exposures works only with RAW captures.
Hope it helps.