PURPOSE: True hybrid cells of heterocaryon are selected from a mixture resulting from fusion of two genetically different kinds of plant cells by adding survical selection of each kind of cells to the cultivation conditions for the callus after fusion.

CONSTITUTION: An N-fixation gene 3 is introduced into one of 2 genetically different kinds of cells 1 and 2 to be fused to form cell 4. Meanwhile, an aerobic N-fixation microorganism is tetatively introduced into the other cell 2 to give cell 6. Thus, a mixed callus containing, in addition to the objective heterocaryon between 4 and 6, homocaryon and not-fused cells 4 and 6 is obtained. Then, the mixed callus are aerobically cultured in a nitrogen-free medium 10 to deactivate N-fixation enzymes in cells 4 and 7 to allow only cells 6, 8 and 9 to be survival. Then, the cultivation is continued in nitrogen-free medium but under anaerobic conditions 11 to keep only cell 9 survival, further the cell 9 is made free from cell 5. Then, the resultant cell 12 is cultured under ordinary conditions 13 to ensure high-efficiency and errorless selection of the objective heterocaryon.