  We present some sufficient conditions for the global in time existence of
solutions of the semilinear Klein-Gordon equation of the self-interacting
scalar field with complex mass. The coefficients of the equation depend on
spatial variables as well, that makes results applicable, in particular, to the
spacetime with the time slices being Riemannian manifolds. The least lifespan
estimate is given for the class of equations including the Higgs boson
equation, which according to physics has a finite lifetime.
