Abstract: A thin film forming apparatus has an evaporation source for emitting a cluster beam, an ionizing device for irradiating the cluster beam from the evaporation source with thermal electrons, thereby forming cluster ions, an accelerating device for accelerating the cluster ions so as to irradiate a substrate with the cluster ions together with neutral clusters, and cluster ion rate control device for periodically varying the rate of irradiation of the substrate with the cluster ions at a period which is longer than the time required for the cluster ions to reach the substrate from the ionizing device and which is shorter than the time required for forming a mono-atom or mono-molecular layer on the substrate, thereby controlling the time-mean of the rate of irradiation with the cluster ions.