The isovector spin monopole resonances (IVSMR) of the β+ type were observed for the first time in the 208Pb and 90Zr(t,3He) reactions at 300 MeV/u. The experiment was performed at the RI Beam Factory (RIBF) at RIKEN by using the newly-constructed magnetic spectrometer SHARAQ. The double differential cross sections for the 208Pb and 90Zr(t,3He) reactions were obtained at the excitation energy of 0≤Ex≤70MeV and the scattering angles of 0°≤θ≤3°. The monopole component was identified by making a difference between the zero-degree and backward-angle spectra. The signatures of the IVSMR are observed at an excitation energy of 12 MeV(20 MeV) in 208Pb(90Zr)(t,3He) reaction, which agrees fairly well with the theoretical predictions.