A forming method and a forming apparatus of nanocrystalline silicon structure makes it possible to prepare a nanocrystalline silicon structure at a low temperature to have densely packed silicon crystal grains which are stably terminated and to effectively control the grain size in nanometer scale. A forming method and a forming apparatus of nanocrystalline silicon structure with oxide or nitride termination, carry out a first step of treating a surface of a substrate with hydrogen radical; a second step of depositing silicon crystals having a grain size of 10 nm or less by the thermal reaction of a silicon-containing gas; and a third step of terminating the surface of the silicon crystal with oxygen or nitrogen by using one of oxygen gas, oxygen radical and nitrogen radical.