  The abrupt structure change from the nuclei of $N=Z \le 35$ to those of $N=Z
\ge 36$ is investigated by means of shell model calculations. The basic
features of the even-even and odd-odd nuclei under consideration are nicely
reproduced. A sudden jump of nucleons into the upper $g_{9/2}d_{5/2}$ shell at
$N=Z=36$ is found to be the main reason that causes the qualitative structure
difference. It is argued that the structure change can be viewed as a decisive
change of the mean field, or a phase transition, along the $N=Z$ line.
