ISAI, Investigating Solar Axion by Iron-57 is an experiment dedicated measurement of an interaction between the axion and nucleus without introducing the other interactions. An event-triggered
monolitic SOI X-ray pixel detector XRPIX, originally developing for the X-ray imaging spec-
troscopy for space observation satellite project, is applied for the Axion experiment with position
sensitive timing veto counters. We developed the readout using a rigid-flex circuit board which
enables us to separate the XRPIX and the most of peripheral electrical components that would
have non-negligible radioactivity without introducing any connectors. We surveyed radioactivity
of the detector inside of the shield using HPGe detector and confirmed a ∼ 10^{3} reduction with
respect to the previous our detector composed of a rigid circuit board. Two timing triggers of the
XRPIX and the veto counter can be recorded by a high time resolution DAQ system so that we will
reject backgrounds induced by cosmic-ray or environmental radiation at the offline analysis. We
constructed the detector with oxygen-free copper and lead shields inside of the climatic chamber.
We will present the experimental apparatus, the performance, the commissioning and the first run.