The present invention relates to the transformation of C4 plants. It also relates to the high-level expression of foreign genes in C4 plants. More specifically, the present invention relates to the creation of C4 plants retaining an excellent photosynthetic capacity at low temperature by achieving high-level expression of an enzyme constituting the C4 photosynthetic pathway. In the present invention, C4 plants are transformed using an expression cassette that comprises a promoter, a C4 plant genomic gene, under control of said promoter, encoding an enzyme constituting a photosynthetic pathway, and a terminator. The C4 plant genomic gene encoding an enzyme constituting a photosynthetic pathway is preferably a C4 plant genome-derived PPDK gene or a modified form thereof. The present invention is particularly useful in improving the production of C4 plants having PPDK (especially, in improving the production of maize under low temperature conditions).