We explored the isothermal crystallization process of poly(3- hydroxybutyrate) by means of simultaneous measurements of time-resolved wide-angle X-ray diffraction (tr-WAXD) and small-angle X-ray scattering (tr-SAXS) methods. The tr-WAXD analyses involve not only (1) a precise analysis of the integral widths but also the analyses such as (2) two-dimensional correlation spectroscopy (2D-COS) and (3) multivariate curve resolution-alternating least squares (MCR-ALS). The tr-SAXS analyses involve not only (4) the conventional one-dimensional correlation function analysis but also the analyses such as (5) 2D-COS between tr-SAXS and tr-WAXD profiles and (6) 2D-COS of tr-SAXS profiles themselves. These analyses elucidated a multistep crystallization process as classified by region I to III in order of the increasing time. In region I, the density fluctuations are first built up in the amorphous matrix, and then the density-rich regions locally develop "intermediate structures" having the mesomorphic orders between pure amorphous melts and pure crystals [lamellar crystallites (LC)], which then grow into layers of the intermediate structures [defined as mesomorphic layers (ML)] with the long spacings. These results were elucidated by analysis (5) and (6). In region II, LC start to be created from ML, which was elucidated by analysis (1) to (4), and both of the weight fractions of ML (X inter) and LC (X crys) increase with time [analysis (3)]. In region III, X inter and X crys decreases and increases with time, respectively [analysis (3)], because the transformation form ML to LC dominates the transformation from the density fluctuations to ML. The WAXD profiles due to ML in region I was identified by analysis (1), while those in regions II and III were identified by analysis (3).