First measurements of plasma parameters in an expander of a mirror trap with sub-fusion plasma parameters are reported. Potential drop near the end plates of the expander appears to be much lower than the electron temperature in the center of the trap. Characteristic energy of electrons in the expander region is also substantially lower than the electron temperature in the confinement plasma. Variation of the magnetic field expansion ratio K between 30 and 160 does not influence the main parameters of the confinement plasma. Electron temperatures of the confinement plasma of about 700 eV were achieved at K = 30.