PURPOSE: To provide a heat-generating material which prevents gas from being evolved, increases the stability of iron powder and is good for long-term storage, by placing a heat-generating mixed powder in an air-barrier bag, said mixed powder consisting of iron powder and a water absorbing powder contg. absorbed NaOH and NaCl.

CONSTITUTION: A heat-generating mixed powder 2 obtd. by mixing iron powder with a water absorbing powder such as activated carbon, charcoal, talc or bentonite, is placed within an air-permeable bag 1. The outside of the bag 1 is sealed by using an air-barrier bag 3. Said water absorbing powder is previously made to absorb an aq. soln. prepd. by adding 0.01W0.5% NaOH to NaCl, KCl or ferric chloride.