  We determine the scale setting function and the pseudo-critical temperature
on the lattice in $N_f=2$ two-color QCD using the Iwasaki gauge and Wilson
fermion actions. Although two-color QCD does not correspond to the real world,
it is very useful as a good testing ground for three-color QCD. The scale
setting function gives the relative lattice spacings of simulations performed
at different values of the bare coupling. It is a necessary tool for taking the
continuum limit. Firstly, we measure the meson spectra for various combinations
of ($\beta,\kappa$) and find a line of constant physics in $\beta$--$\kappa$
plane. Next, we determine the scale setting function via $w_0$ scale in the
gradient flow method. Furthermore, we estimate the pseudo-critical temperature
at zero chemical potential from the chiral susceptibility. Combining these
results, we can discuss the QCD phase diagram in which both axes are given by
dimensionless quantities, namely, the temperature normalized by the
pseudo-critical temperature on the lattice and the chemical potential
normalized by the pseudoscalar meson mass. It makes it easy to compare among
several lattice studies and also makes it possible to compare theoretical
analyses and lattice studies in the continuum limit.
