Lattice constants $a$ and $c$ were measured in the mixed crystals $\mathrm{Ag}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{In}}_{x}{\mathrm{Se}}_{2}$ for $0\ensuremath{\le}x\ensuremath{\le}1$. They showed a miscibility region from $x=0.0$ to $x=0.4$ and from $x=0.8 \mathrm{to} 1.0$, and then immiscibility near $x=0.6$ in the composition dependence of the lattice constants $a$ and $c$. Also, the photoconductivity peak energy and the energy gap measured in the composition range from $x=0.0 \mathrm{to} 1.0$ showed a discontinuity near $x=0.6$. The temperature coefficient of the photoconductivity peak energy is (1.4-2.8) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$ eV/K and that of the energy gap (2.3-3.8) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$ eV/K in the temperature range 25-75 K. The sign of the temperature coefficient reverses and the values are -(2.4-4.1) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$ and -(2.4-5.1) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$ eV/K, respectively, in the temperature range 100-300 K.