Multiple melting peaks of high-density polyethylene produced by complex thermal pretreatment are investigated with variable temperature Raman spectroscopy and differential scanning calorimetry (DSC). We examine the origin of the observed multiple melting peaks with three possible model mechanisms: melting-recrystallization, pre-melting via mesophases, and melting of multithickness lamellae. We quantitatively analyze Raman spectral change associated with each melting peak with respect to temperature. The Raman spectral change is represented by a least squares moving-window (LSMW) method, which is effective in suppression of high-frequency derivative noise. The conformational similarity among multiple melting, peaks strongly suggests that dominant melting occurs from the crystalline phase directly to the amorphous phase at all transitions. The observed multiple melting transitions are consistent with one-step melting of multithickness lamellae as the dominant origin. However, two-dimensional Raman correlation spectroscopy suggests that noncrystalline trans-rich conformations appear during the melting process, whose mass fraction is estimated as 10% or less. The combined techniques are shown to provide better insights into the melting process.