There are provided a Si alloy powder for a lithium ion secondary battery negative electrode active material, which has a high discharge capacity and excellent cycle life, and a method for producing the same. The Si alloy powder of the present invention comprises a eutectic structure including a Si phase and a CrSi2 phase, and the average value of thicknesses in a thin width direction in each phase of the Si phase and the CrSi2 phase is 4 μm or less. This Si alloy powder is produced by quenching and solidifying a dissolution material which gives the composition of the Si alloy powder at a cooling rate of 100° C./s or more.