  Interstellar magnetic fields exist over a broad range of spatial scales,
extending from the large Galactic scales ($\sim 10$ kpc) down to the very small
dissipative scales ($\ll 1$ pc). In this paper, we use a set of 490 pulsars
distributed over roughly one third of the Galactic disk out to a radius $R
\simeq 10$ kpc (assuming $R_\odot = 8.5$ kpc) and combine their observed
rotation and dispersion measures with their estimated distances to derive the
spatial energy spectrum of the Galactic interstellar magnetic field over the
scale range $0.5 - 15$ kpc. We obtain a nearly flat spectrum, with a 1D
power-law index $\alpha=-0.37\pm0.10$ for $E_{\rm B}(k)=C k^{\alpha}$ and an
rms field strength of approximately $6 \mu$G over the relevant scales. Our
study complements the derivation of the magnetic energy spectrum over the scale
range $0.03 - 100$ pc by \citet{ms96b}, showing that the magnetic spectrum
becomes flatter at larger scales. This observational result is discussed in the
framework of current theoretical and numerical models.
