The quantum spin Hall effect (QSHE) in three-dimensional topological insulators (3D-TIs) is an important hallmark of realization of ultralow dissipative electrical transport originating from nontrivial topological surface Dirac states (TSDSs). The observation of QSHE, however, had been limited at low temperatures for thin film crystals prepared by epitaxial growth or ultrathin (nanometer-level) flakes, and the finite Fermi energy tuning was required using field effect transistor (FET) construction. Here, we show that a clear observation of QSHE with a quantized level of e 2 / h can be possible for high quality single crystal flakes of Sn-(Bi,Sb)2(Te,S)3 (Sn-BSTS) prepared by an exfoliation method without any gate control under FET construction. The QSHE is observed in an extremely large sample area of millimeter square at the largest thickness of micrometer and up to the highest temperature of 20 K, which is greatly superior to those reported for other 3D-TIs so far. The accurate band picture of both top and bottom topological surface states of Sn-BSTS is determined from both the QSHE and the Shubnikov-de Hass (SdH) oscillations. High quality Sn-BSTS of 3D-TIs will effectively be used for experiments targeting on science as well as many technological applications expected in the future.