To permit the pull-out of a core simply even when placed concrete is set and contracted by a method wherein the core for manufacturing a hollow concrete block is formed of a tubular shape memory alloy of iron series.

A core 2, employed for manufacturing a hollow concrete block, is formed of a hollow square tubular unit 2a and a ceiling plate 2b, which are formed of plate members made of a shape memory alloy of iron series. The core 2 is formed so as to have a size, smaller than a size necessary for using it as the core 2 by several % or about 1%, for example. Heat treatment at 600°C is applied to memorize the shape of the core 2. Upon using the core, an internal pressure is applied so that the size of the core 2 coincides with the actual size of an actual product. Then, concrete 5 is poured and set, then, a heater 4 is set in the core 2 to heat the same and effect the restoring treatment of configuration. According to this method, the size of the core 2 is returned to the initial small size and the core 2 can be pulled out easily.