A method for forming a thin film of cerium oxide as a blocking layer which constitutes a portion of a photoelectric film of a blocking type image pickup tube is disclosed. A substrate deposition rate in a vacuum deposition process is established in a range between 0.01 to 0.6 A/sec to prevent the deposition of particles which result in black or white spots in a picture image. It is more effective to select a particle size of 5 .mu. or more for the primary particles to be deposited.