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We present electron densities $n_{\rm e}$ in the inter-stellar medium (ISM)of star-forming galaxies at $z=4-9$ observed by the JWST/NIRSpec GLASS, ERO,and CEERS programs. We carefully evaluate line-spread functions of the NIRSpecinstrument as a function of wavelength with the calibration data of a planetarynebula taken onboard, and obtain secure [OII]$\lambda\lambda$3726,3729 doubletfluxes for 14 galaxies at $z=4.02-8.68$ falling on the star-formation mainsequence with the NIRSpec high and medium resolution spectra. We thus derivethe electron densities of singly-ionized oxygen nebulae with the standard$n_{\rm e}$ indicator of [OII] doublet, and find that the electron densities ofthe $z=4-9$ galaxies are $n_{\rm e}\gtrsim 300$ cm$^{-3}$ significantly higherthan those of low-$z$ galaxies at a given stellar mass, star-formation rate(SFR), and specific SFR. Interestingly, typical electron densities of singlyionized nebulae increase from $z=0$ to $z=1-3$ and $z=4-9$, which isapproximated by the evolutionary relation of $n_{\rm e}\propto(1+z)^{p}$ with$p\sim 1-2$. Although it is not obvious that the ISM property of $n_{\rm e}$ isinfluenced by global galaxy properties, these results may suggest that nebuladensities of high-$z$ galaxies are generally high due to the compactmorphologies of high-$z$ galaxies evolving by $r_{\rm e}$ approximatelyproportional to $(1+z)^{-1}$ ($r_{\rm vir} \propto (1+z)^{-1}$) for a givenstellar (halo) mass whose inverse square corresponds to the $p\sim 2$evolutionary relation. The $p\sim 1-2$ evolutionary relation can be explainedby a combination of the compact morphology and the reduction of $n_{\rm e}$ dueto the high electron temperature of the high-$z$ metal poor nebulae.

Redshift Evolution of the Electron Density in the ISM at $z\sim 0-9$ Uncovered with JWST/NIRSpec Spectra and Line-Spread Function Determinations