An attempt was made to clarify the ionization mechanism and to interpret the characteristics of the measured Hg+ion density in low‐pressure Ar‐Hg discharge for the mercury lamp using the measured Hg6p3P0,1,2population density, electron density, and electron temperature. The stepwise ionization caused by collisions between Hg metastable atoms and electrons and that between Hg excited atoms were concluded to be the most important ionization processes. As the extinction processes of Hg+ions, the ambipolar diffusion and recombination of Hg atoms, Hg+ions and electrons were taken into account. The measured dependencies of the Hg+ion density on bath temperature was interpreted well with the model including the above processes.