The following work presents the results of investigation of microinstabilities in the anisotropic synthesized hot ion plasmoid (SHIP). Plasmoid is located in a small mirror section that is installed at one side of the GDT facility, which is an axially symmetric magnetic mirror device of gas dynamic trap type. To define the type and the parameters of the developing microinstability a set of high-frequency electrostatic and magnetic probes was used. The microinstability observed in the additional section of GDT is the Alfven ion cyclotron instability (AIC), because of small azimuthal wave numbers, magnetic field vector rotating in the direction of ion gyration and oscillation frequency below the actual ion cyclotron frequency. AIC instability threshold was registered at the following plasma parameters: fast ion density n > 3 × 1013 cm-3, ratio of ion pressure to magnetic field pressure [approximately equal] 0.02, anisotropy A = 40, ai/Rp [approximately equal] 0.23, where ai is the ion gyroradius and Rp is the plasmoid radius.