  We investigated the pressure effects on the FeSe superconductor and the
related compounds. Pressure dependence of superconducting transition
temperature (Tc) for FeSe0.8S0.2 exhibits a dome-shaped behavior below 0.76
GPa. On the other hand, the Tc of FeSe0.25Te0.75 linearly increases up to 0.99
GPa. Here we discuss the relation between the physical pressure effects and the
chemical pressure effects on the FeSe system.
