A method for producing an active material particle for a lithium ion battery, the method including steps of: flowing a plurality of raw material solutions into respective raw material-feeding channels under a pressure of 0.3 to 500 MPa, the solutions being capable of inducing a chemical reaction when mixed, thereby producing an active material particle for a lithium ion battery or an active material precursor particle for a lithium ion battery; and mixing the plurality of raw material solutions at a junction of the raw material-feeding channels to induce the chemical reaction, thereby continuously producing an active material particle for a lithium ion battery or producing an active material precursor particle for a lithium ion battery.