We present a study of Shubnikov-de Haas (SdH) oscillations at temperatures of (2.2-10) K in magnetic fields up to 2.5 T in the HgCdTe/HgTe/HgCdTe heterostructure for a wide (20.3 nm) HgTe quantum well with an inverted energy band structure. The analysis of the temperature dependence of SdH amplitude in weak fields, in a region of doubly degenerate magnetoresistance peaks, led us to the value of effective electron mass mc/m0=(0.022±0.002) which is about half the theoretical estimates. But in a region of higher magnetic fields, for nondegenerate magnetoresistance peaks, we confidently have mc/m0=(0.034±0.003) in good agreement both with the theoretical estimation and with our experimental results on the analysis of activation transport under quantum Hall effect regime. The reasons for this discrepancy are discussed. Keywords: quantum wells, mercury telluride, Shubnikov-de Haas oscillations, effective mass of charge carriers, quantum Hall effect.