The object of the present invention is to provide transgenic plants which accumulate free amino acids, particularly at least one amino acid selected from among glutamic acid, asparagine, aspartic acid, serine, threonine, alanine and histidine accumulated in a large amount, in the edible parts thereof, and a method of producing them are provided. The other object of the present invention is to provide a method of increasing the yielding of potato and to provide a transgenic potato of which yielding can be increased. In the present invention, a sequence encoding glutamate dehydrogenase (GDH) gene is introduced into a plant together with a suitable regulatory sequence to express it in a plant cell, and thereby the GDH gene is excessively expressed.