The purpose of the present invention is to improve initial charge-discharge efficiency and cycle properties in a non-aqueous electrolyte secondary battery in which a silicon material is used as a negative electrode active material. A negative electrode active material particle (10), which is one embodiment, has a lithium silicate phase (11) represented by Li2zSiO(2+z) {0 < z < 2} and silicon particles (12) dispersed in the lithium silicate phase (11). Each of the silicon particles (12) has a crystallite size of 40 nm or less. The crystallite size can be calculated from a half-width value of a diffraction peak of an Si (111) plane of an XRD pattern obtained by the XRD measurement of the negative electrode active material particles (10) in accordance with the Scherrer equation.