  Intelligent reflecting surface (IRS) is envisioned to be a promising solution
for designing spectral and energy efficient wireless systems. In this letter,
we study a multi-IRS-assisted system under Rician fading where the phase shifts
adapt to only the line of sight (LoS) components. First, we analyze and
optimize the outage probability of the multi-IRS-assisted system in the slow
fading scenario for the non-LoS (NLoS) components. We also show that the
optimal outage probability decreases with the numbers of IRSs and elements of
each IRS when the LoS components are stronger than the NLoS ones. Then, we
characterize the asymptotically optimal outage probability in the high
signal-to-noise ratio (SNR) regime, and show that it decreases with the powers
of the LoS components. To the best of our knowledge, this is the first work
that studies the outage probability in multi-IRS-assisted systems.
