A method of manufacturing a bone cutting assist device used for cutting a bone deformed into an abnormal condition, the method comprising: acquiring three-dimensional data of a bone to be treated and generating a three-dimensional bone model representing the bone to be treated, on a basis of the acquired three-dimensional data: generating bone piece models (BPM1, BPM2) divided by cutting the bone model along a cutting cross-section for correction defined with respect to the bone model, the bone piece models (BPM1, BPM2) being movable or rotatable to a target correction position approximate to a target bone model representing a correction goal of the bone to be treated; calculating a position of a first rod model representing a shape of a first rod to be used for securing that the bone cutting assist device is located at a planned position on the bone to be treated, the first rod model being attached to a feature point of the bone model, the feature point being determined as a position where the first rod model is to be thrust into and showing a shape portion that an operator can visually recognize easily in a shape portion of the bone model; calculating, on a basis of a position of a second rod model attached to each of the bone piece models (BPM1, BPM2) assumed to be located at the target correction position, a position of the second rod model on an assumption that the bone piece models (BPM1, BPM2) are located at a position corresponding to the bone model; generating a bone cutting assist device model representing a bone cutting assist device including a fitting surface to be fitted to a surface of the bone model, a cutting slit formed at a position corresponding to the cutting cross-section, to guide a cutting jig toward the cutting cross-section, a first guide hole formed at the calculated position of the first rod model, so as to allow the first rod model to be inserted, and a second guide hole formed at the calculated position of the second rod model, so as to allow the second rod model to be inserted; outputting three-dimensional manufacturing data representing the generated bone cutting assist device model; and manufacturing the bone cutting assist device according to the outputted three-dimensional manufacturing data. generating bone piece models (BPM1, BPM2) divided by cutting the bone model along a cutting cross-section for correction defined with respect to the bone model, the bone piece models (BPM1, BPM2) being movable or rotatable to a target correction position approximate to a target bone model representing a correction goal of the bone to be treated; calculating a position of a first rod model representing a shape of a first rod to be used for securing that the bone cutting assist device is located at a planned position on the bone to be treated, the first rod model being attached to a feature point of the bone model, the feature point being determined as a position where the first rod model is to be thrust into and showing a shape portion that an operator can visually recognize easily in a shape portion of the bone model; calculating, on a basis of a position of a second rod model attached to each of the bone piece models (BPM1, BPM2) assumed to be located at the target correction position, a position of the second rod model on an assumption that the bone piece models (BPM1, BPM2) are located at a position corresponding to the bone model; generating a bone cutting assist device model representing a bone cutting assist device including a fitting surface to be fitted to a surface of the bone model, a cutting slit formed at a position corresponding to the cutting cross-section, to guide a cutting jig toward the cutting cross-section, a first guide hole formed at the calculated position of the first rod model, so as to allow the first rod model to be inserted, and a second guide hole formed at the calculated position of the second rod model, so as to allow the second rod model to be inserted; outputting three-dimensional manufacturing data representing the generated bone cutting assist device model; and manufacturing the bone cutting assist device according to the outputted three-dimensional manufacturing data.