Using an idea of an "available state" on state-transition diagrams used in continuous-time Markov analysis to absorb the variety of designed control functions, we present a general framework of availability analysis of control systems according to IEC standards, which is based on continuous-time Markov analysis and gives the value of steady state availability as the analysis result. It enables us to quantitatively evaluate fault tolerance of control systems in various application areas, and thus it succeeds in improving the practicality of fault-tolerant control.