The phase relations and pressure volume dependences of ${\mathrm{ZrO}}_{2}$ under high pressure and high temperature have been investigated by means of in situ observation using multianvil-type high-pressure devices and synchrotron radiation. By compression of 3--4 GPa, baddeleyite (monoclinic ${\mathrm{ZrO}}_{2})$ transforms to two distorted fluorite $({\mathrm{CaF}}_{2})$-type phases depending on temperature: an orthorhombic phase, orthoI, below $600\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and a tetragonal phase above $600\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ Both orthoI and tetragonal phases then transform into another orthorhombic phase, orthoII, with a cotunnite $({\mathrm{PbCl}}_{2})\ensuremath{-}\mathrm{t}\mathrm{y}\mathrm{p}\mathrm{e}$ structure above 12.5 GPa and the phase boundary is almost independent of temperature. OrthoII is stable up to $1800\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and 24 GPa. The unit-cell parameters and the volumes of these high-pressure phases have been determined as functions of pressure and temperature. The orthoI/tetragonal-to-orthoII transition accompanies about 9% volume decrease. The thermal expansion coefficient of orthoII at 20 GPa is $2.052\ifmmode\pm\else\textpm\fi{}0.003\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}{\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$ over $25--1400\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ The bulk modulus calculated using Birch-Murnaghan's equations of state is 296 GPa for orthoII, which suggests that the high-density ${\mathrm{ZrO}}_{2}$ is a candidate for potentially very hard materials. The phase relation of stabilized cubic ${\mathrm{ZrO}}_{2},$ ${\mathrm{C}\mathrm{a}\mathrm{O}\ensuremath{-}\mathrm{Z}\mathrm{r}\mathrm{O}}_{2},$ under pressure at elevated temperature has also been examined. Distorted fluorite-type phases do not appear in ${\mathrm{C}\mathrm{a}\mathrm{O}\ensuremath{-}\mathrm{Z}\mathrm{r}\mathrm{O}}_{2}$ but the direct transition from cubic phase to orthoII is observed on the same P-T conditions as in pure ${\mathrm{ZrO}}_{2}.$