An atom‐cluster generator which can be set in the specimen chamber of a transmission electron microscope has been designed and constructed. The generator essentially consists of two vacuum chambers; one is the chamber for differential pumping and the other the cluster source chamber. The former plays an essential role in keeping the column of a microscope in a high vacuum. The latter is kept in a gas atmosphere and nm‐sized metal clusters are produced in this chamber by evaporating a metal element from a tungsten filament. With this apparatus, preliminaryinsitudeposition experiments were carried out, and it was observed that isolated gold clusters, ejected from the atom‐cluster generator, landed sequentially on a substrate.