  The Aerogel Ring Imaging Cherenkov (ARICH) counter serves as a particle
identification device in the forward end-cap region of the Belle II
spectrometer. It is capable of identifying pions and kaons with momenta up to
$4 \, {\rm GeV}/c$ by detecting Cherenkov photons emitted in the silica aerogel
radiator. After the detector alignment and calibration of the probability
density function, we evaluate the performance of the ARICH counter using early
beam collision data. Event samples of $D^{\ast +} \to D^0 \pi^+ (D^0 \to
K^-\pi^+)$ were used to determine the $\pi(K)$ efficiency and the $K(\pi)$
misidentification probability. We found that the ARICH counter is capable of
separating kaons from pions with an identification efficiency of $93.5 \pm 0.6
\, \%$ at a pion misidentification probability of $10.9 \pm 0.9 \, \%$. This
paper describes the identification method of the counter and the evaluation of
the performance during its early operation.
