A decrease in emission intensity owing to predissociation in the Fulcher-α band (d 3Π u → a 3Σ g +) in the d 3Π u(ν′ = 4) vibronic state of H 2 was investigated. The measured population in the d 3Π u(ν′ = 4) state, which is above the dissociation limit to H(1s) and H(2s), was compared with that predicted from d 3Π u(ν′ = 0-3) emission, which does not suffer predissociation. We measured the pressure dependence of the Fulcher-α band spectra in a hollow-cathode glow discharge apparatus. From the linear dependence of collisional quenching rate on gas pressure, predissociation rate was determined, and it was verified by applying it to low-pressure arc discharge plasmas of < 2 Pa in the divertor simulator MAP-II, in which collisional quenching can be neglected. ©2006 The Physical Society of Japan.