The structures in spherulites of poly(butylene succinate) (PBSU) are discussed by analyzing the spherulitic growth rate of poly(ethylene oxide) (PEO) inside PBSU spherulites of PBSU/PEO blends, which are binary blends of crystalline polymers where PBSU spherulites fill the whole volume before PEO spherulites nucleate and grow inside PBSU spherulites. The results indicated that a PBSU spherulite contains wide amorphous regions and that PBSU crystals suppress the molecular mobility of PEO very little. It also implied that PEO crystallizes primarily in interfibrillar regions in PBSU spherulites. The same equation as that for crystalline/amorphous blends was applicable to the dependence of the spherulitic growth rate of PEO, G_PEO, on the crystallization temperature by using the same activation energy for molecular transport as in a melt. The variation of G_PEO with the blend composition could be ascribed to the volume fraction of PEO in the amorphous region of PBSU spherulites.