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Although a number of molecular complexes have been studied extensively for a long time, studies on open-shell radical complexes are relatively scarce due to its difficulty of production. Besides, most of them deal with relatively simple rare-gas(Rg)-diatomic systems. We report the first microwave observation of the Rg-open-shell triatomic species, $Ar-HO2$. The a-type and b-type transitions of $Ar-HO2$ have been observed using Fourier transform microwave (FTMW) and FTMW-millimeter wave double resonance spectroscopy. The $r0$ structure of $Ar-HO2$ has been determined from the rotational constants by fixing the structure of the $HO2$ monomer. The determined molecular structure is cis planar, where the argon atom is attached to the hydrogen atom of $HO2;r0(ArH)=2.622\backslash AA$, and $\theta 0(ArHO)=153.9\circ$. $r0(ArH)$ of $Ar-HO2$ is shoter than that of Ar-HO, $2.776\backslash AAa$, indicating that the $Ar-HO2$ bond is relatively rigid. In fact, the binding energy of $Ar-HO2,292cm-1$, calculated by RCCSD(T)/aug-cc-pVTZ + bond function is much larger than that of Ar-HO or other closed-shell Rg-triatomic species. On the other hand, the determined fine and hyperfine structures are almost identical to those of the $HO2$ monomer. The large binding energy may be ascribed to the effect of the low-lying first excited state of the $HO2$ monomer.