To provide a negative active material for a lithium ion secondary battery having high discharge capacity per volume and high cycle characteristics and to provide its manufacturing method.

This negative active material for the lithium ion secondary battery has alloy composition represented by formula (1), is composed of alloy structure containing practically no single phase of Sn, and the manufacturing method of the new negative active material contains a process mixing raw materials forming the composition of the formula (1) and melting to form molten metal; a process cooling the obtained molten metal to a temperature of solidification point or less at a cooling speed of 100°C/s or more for solidifying; and a process keeping the obtained solidified material in a temperature range of 250-700°C. RxMaySnzMbw(1) (R: Y, a rare-earth element; Ma: Mn, Co, or the like; Mb: Al, Si; 0.20≤x≤1.00; 0≤y≤0.80; 2.50≤z≤4.50; 0≤w≤1.00; x+y=1; 2.50≤z+w≤4.50).

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