In order to investigate extremely slow fatigue crack growth characteristics of JIS SCM440 CT specimen in 9 MPa hydrogen gas environment, stress intensity factor range (ΔK) decreasing tests with in-situ observation were carried out. Fatigue crack growth rate (FCGR) in hydrogen gas did not show threshold behavior but FCGR in helium and air showed threshold behavior clearly. Fatigue crack in hydrogen gas showed sudden increased after a temporary stop in growth. The sudden increase in growth was induced by coalescing with a new micro-crack initiated in front of the main crack tip. The fractographic analysis showed the existence of intergranular facets. The intergranular facets were observed in all over the fracture surface. The amount of the intergranular facets in hydrogen gas decreased with decreasing the ΔK. However, facets were still observed in the extremely low rate region. On the other hand, there was no facet in fracture surface tested in helium and in air in the extremely low rate region. The formation of facets was supposed to be one of the causes of non-threshold behavior in hydrogen gas.