  The free-free absorption of low frequency radio waves by thermal electrons in
the warm ionized medium of our Galaxy becomes very significant at $\lesssim 10$
MHz (ultralong-wavelength), and the absorption strength depends on the radio
frequency. Upcoming space experiments such as the Discovering Sky at the
Longest wavelength (DSL) and Farside Array for Radio Science Investigations of
the Dark ages and Exoplanets (FARSIDE) will produce high-resolution
multi-frequency sky maps at the ultralong-wavelength, providing a new window to
observe the Universe. In this paper we propose that from these
ultralong-wavelength multi-frequency maps, the three-dimensional distribution
of the Galactic electrons can be reconstructed. This novel and robust
reconstruction of the Galactic electron distribution will be a key science case
of those space missions. Ultralong-wavelength observations will be a powerful
tool for studying the astrophysics relevant to the Galactic electron
distribution, for example, the impacts of supernova explosions on electron
distribution, and the interaction between interstellar atoms and ionizing
photons escaped from the HII regions around massive stars. An animation shows
the reconstructed results using {\tt NE2001} model as input test. On ArXiv, it
is given in the directory: Ancillary files. In the paper the animation is
linked to Fig. 5.
