The present invention provides a method for Agrobacterium-mediated gene introduction into a plant material, comprising: 1) treating the plant material, and then 2) infecting the plant material with an Agrobacterium, characterized in that a medium enriched in a metal salt containing copper ion is used in step 1) and/or 2). The present invention also provides a process for preparing a transformed plant characterized in that the gene introduction method of the present invention is used.