In subjecting a substrate of low electric conductivity to a pretreatment by holding the same within a fluidized bed of diamond particles for the formation of a diamond film on the substrate surface, a treatment method is disclosed which is capable of preventing decrease in the effect of the pretreatment. The pretreatment of the substrate is conducted within a fluidized bed of diamond particles by keeping the electrostatic potential of the substrate in the range from −1.5 to +1.5 kV. It is desirable that the relative humidity of the gas for the fluidization of diamond particles is controlled to be 40% or higher. It is more desirable that ion bombardment onto the substrate surface to effect neutralization of the electrostatic charges. The fluidizing gas is humidified preferably by using a bubbling apparatus or spraying apparatus.