To reexamine the possibility of the spin-forbidden, molecular-elimination mechanism of the thermal decomposition of H2S, H2S + M → S(3P) + H2 + M (1), recently indicated by experimental and theoretical studies, as well as to examine the cause of discrepancies of its rate constants among recent works, the reverse insertion channel of the S(3P) + H2 reaction, S(3P) + H2 + M → H2S + M (-1), has been investigated. The experiments have been conducted with an excimer-laser photolysis (248 nm) in a shock tube at a lower temperature range, 900-1050 K, and a higher pressure range up to 4 atm, than previous studies, where the insertion process (-1) was estimated to be dominant over the simple hydrogen-atom-transfer reaction, S(3P) + H2 → H + HS (2). The decay rate of S(3P) atoms has been measured by using an atomic resonance absorption spectrometry technique. The measured rate constant agreed well with the extrapolation of the previous measurements for reaction 2, showing that the recombination channel (-1) is still minor at these experimental conditions. The upper limit of the rate constant for reaction 1 derived in the present study was shown to be consistent with the theoretical rate constant calculated by the Troe's formula with the ab initio threshold energy for the intersystem crossing and with reasonable weak collision factors derived from the rate constant for reaction 1 reported by Shiina et al.