[en] A practical method of calculating the pionic decay strength function for the three-body final states is presented on the basis of the Kapur-Peierls treatment of the continuum state. The results for the Λ⁵He decay are in good agreement with the experiment. The application to the _ΛΛ⁶He decay results in the pion spectra which have a discrete or almost monoenergetic peak, showing the experimental feasibility of using successive pions as an indicator of double-Λ hypernuclear production. The pionic decay rates are also evaluated for another typical hypernucleus _Λ⁴H which is very abundantly produced in the CK- stopped, _π^% reaction. The interplay between the two-body decay channel (discrete pion momentum) and the three-body continuum channel has been demonstrated. It is emphasized that the hypernuclear pionic decay is a useful probe to know the Λ state in the nucleus. (orig.)