A pharmaceutical composition for prevention and/or treatment for HIV infection or AIDS induced by the infection which comprises, as an active ingredient, a triazaspiro[5.5]undecane derivative, a quaternary ammonium salt thereof, an N-oxide thereof, or a non-toxic salt thereof, and if necessary, it may be combined with at least one member of other agents for prevention and/or treatment for HIV infection (wherein all symbols are as defined in the specification.) ##STR00001## The triazaspiro[5.5]undecane derivatives, the quaternary ammonium salts thereof or the N-oxides thereof, or the non-toxic salts thereof are useful in preventing and/or treating HIV infection and AIDS induced by the infection.