The elastic-plastic fracture toughness, J_Ic, of SM490B carbon steel plate was investigated in air and 0.7 MPa hydrogen gas. J_Ic tests were conducted in accordance with the JSME standard, JSME S001 (1981). J_Ic was much smaller in hydrogen at a displacement velocity of V = 2 × 10^-3 mm/s (J_Ic = 10.0 kJ/m²) than in air at V = 2 × 10^-3 mm/s (J_Ic = 248.6 kJ/m²). J_Ic in air does not satisfy the validity requirement. In hydrogen, surprisingly, a further decrease in V did not decrease J_Ic, but increased it. J_Ic in hydrogen at V = 2 × 10^-5 mm/s was 60.9 kJ/m². The large and small values of J_Ic in air and hydrogen corresponded to the fracture morphology. In air at V = 2 × 10^-3 mm/s, a critical stretched zone, SZW_c, was formed at the tip of the fatigue pre-crack, followed by dimples. In hydrogen at V = 2 × 10^-3 mm/s, quasi-cleavage instead of SZW_c and dimples were formed at the pre-crack tip. In hydrogen at V = 2 × 10^-5 mm/s, SZW_c was formed at the pre-crack tip, followed by dimples again. This elastic-plastic fracture toughness behavior was analyzed assuming HESFCG (hydrogen-enhanced successive fatigue crack growth), which is proposed by the authors to explain the acceleration of fatigue crack growth rate in the presence of hydrogen. The elastic plastic fracture toughness test shown in 0.7 MPa hydrogen gas at V = 2 × 10^-3 mm/s is the same as that shown in a fatigue crack growth test in 0.7 MPa hydrogen gas at a number of cycles of n = 1 and stress ratio of R = 0; and thus J_Ic in 0.7 MPa hydrogen gas at V = 2 × 10^-3 mm/s is not the real elastic-plastic fracture toughness. We conclude that the real elastic-plastic fracture toughness in 0.7 MPa hydrogen gas can be determined by fracture toughness testing in 0.7 MPa hydrogen gas at V = 2 × 10^-5 mm/s.