We have detected diffuse X-ray emission within the planetary nebula shell of Abell 30, using a 21.8 ks ROSAT Position Sensitive Proportional Counter observation. The spectrum is very soft, with almost all source counts at energies below 0.4 keV. The extended emission is real because most of the detected counts are within the 0.2--0.4 keV band, which is not seriously plagued by the electronic ghost-image problem. The best-fit Raymond & Smith plasma emission models give plasma temperatures of (2--4.5) x 105 K. The X-ray emission could arise from the hot shocked stellar wind or the central H-poor nebula shocking the ambient H-rich material. Low plasma temperatures are expected in shocked stellar wind, if there is a strong heat conduction at the interface between the hot shocked wind and the surrounding cool nebular shell, or if the fast stellar wind is mass-loaded by the ejecta clumps. Low plasma temperatures are also expected behind the moderate shock of the H-poor shell advancing into the H-rich envelope. High-resolution X-ray images of A30 are needed to determine more accurately the relationship between the X-ray emission and the nebular shells.