  The supernova SN 1978K has been noted for its lack of emission lines broader
than a few thousand km/s since its discovery in 1990. Modeling of the radio
spectrum of the peculiar SN 1978K indicates the existence of HII absorption
along the line of sight. To determine the nature of this absorbing region, we
have obtained a high-dispersion spectrum of SN 1978K at the wavelength range
6530--6610 \AA. The spectrum shows not only the moderately broad H-alpha
emission of the supernova ejecta but also narrow nebular H-alpha and [N II]
emission. The high [N II]6583/H-alpha ratio, 0.8-1.3, suggests that this radio
absorbing region is a stellar ejecta nebula. The expansion velocity and
emission measure of the nebula are consistent with those seen in ejecta nebulae
of luminous blue variables. Previous low-dispersion spectra have detected a
strong [N II]5755 line, indicating an electron density of 3-12x10^5 cm^{-3}. We
argue that this stellar ejecta nebula is probably part of the pre-shock dense
circumstellar envelope of SN 1978K. We further suggest that SN 1997ab may
represent a young version of SN 1978K.
