Pseudomonas aeruginosa is an indigenous bacteria which inhabits on the surfaces of aqueous environments and is representative pathogen that causes opportunistic infection. P.aeruginosa is classified as low-virulence organism because it rarely causes infection in immunocompetent hosts, but in immunocompromised hosts it can cause a variety of severe infections leading to mortality. P.aeruginosa is also a representative pathogen of nosocomial infection. Emerging multidrug-resistant P.aeruginosa is now causing serious problems on clinical site. Regarding virulence factors, increasing number of studies using gene disrupted mutant bacterial strains revealed that P.aeruginosa possesses a variety of machinaries such as type 3 secretion system, exoenzyme, biofilm formation that impairs or evades host immune system. Also it is getting clarified that the virulence of P.aeruginosa is controlled by an inter-bacterial signal transduction system called quorum sensing. Meanwhile, studies on the host defense system using various gene targeting mice revealed the mechanisms by which host immune system detects and defends against invasion of P.aeruginosa. In this review, we describe these virulence factors, the mechanisms impairing host immune system, host's pathogen recognition system and immune evasion by P.aeruginosa.