A method for designing cutting conditions that designs cutting conditions required for cutting a workpiece with a cutting tool, characterized by comprising: calculating a deflection amount (α) of the cutting tool by using design parameters comprising a feed speed (f) of the cutting tool, an axial direction cutting amount (da) by the cutting tool, a radial direction cutting amount (dr) by the cutting tool, and a cutting speed (v) of the cutting tool; determining whether a "chattering vibration" occurs or not in the cutting tool during the cutting based on the calculated deflection amount (α) of the cutting tool and a predetermined threshold (β); calculating a maximum cutting thickness (Ct max) by the cutting tool, which is an allowed cuttable maximum thickness of the workpiece, from the feed speed (f) of the cutting tool and the radial direction cutting amount (dr) by the cutting tool, when it is determined that the "chattering vibration" does not occur in the cutting tool; calculating a cutting temperature (t) of the cutting tool from the calculated maximum cutting thickness (Ct max), and the cutting speed (v) of the cutting tool; determining whether a tool life of the cutting tool in the cutting is satisfied or not based on the calculated cutting temperature of the cutting tool and a predetermined threshold; calculating a cutting efficiency (e) of the cutting tool in the cutting from a maximum value of the cutting speed of the cutting tool allowed in the cutting, the feed speed (f) of the cutting tool, the axial direction cutting amount (da) by the cutting tool, and the radial direction cutting amount (dr) by the cutting tool, when the tool life of the cutting tool is satisfied; and comparing the calculated cutting efficiency (e) of the cutting tool with data of a cutting efficiency stored in advance and when the calculated cutting efficiency (e) of the cutting tool is a maximum value among the data of the cutting efficiency (e), using, as the cutting conditions, the feed speed (f) of the cutting tool, the axial direction cutting amount by the cutting tool, the radial direction cutting amount (dr) by the cutting tool, and the cutting speed (v) of the cutting tool, wherein the deflection amount (α) of the cutting tool is calculated by the following expression: α = Fb ave / G, wherein α is the deflection amount (m), Fb ave is an average value of a cutting resistance in a vibrating direction of the cutting tool, and G is a bending rigidity (N/m) in the vibrating direction of the cutting tool. calculating a deflection amount (α) of the cutting tool by using design parameters comprising a feed speed (f) of the cutting tool, an axial direction cutting amount (da) by the cutting tool, a radial direction cutting amount (dr) by the cutting tool, and a cutting speed (v) of the cutting tool; determining whether a "chattering vibration" occurs or not in the cutting tool during the cutting based on the calculated deflection amount (α) of the cutting tool and a predetermined threshold (β); calculating a maximum cutting thickness (Ct max) by the cutting tool, which is an allowed cuttable maximum thickness of the workpiece, from the feed speed (f) of the cutting tool and the radial direction cutting amount (dr) by the cutting tool, when it is determined that the "chattering vibration" does not occur in the cutting tool; calculating a cutting temperature (t) of the cutting tool from the calculated maximum cutting thickness (Ct max), and the cutting speed (v) of the cutting tool; determining whether a tool life of the cutting tool in the cutting is satisfied or not based on the calculated cutting temperature of the cutting tool and a predetermined threshold; calculating a cutting efficiency (e) of the cutting tool in the cutting from a maximum value of the cutting speed of the cutting tool allowed in the cutting, the feed speed (f) of the cutting tool, the axial direction cutting amount (da) by the cutting tool, and the radial direction cutting amount (dr) by the cutting tool, when the tool life of the cutting tool is satisfied; and comparing the calculated cutting efficiency (e) of the cutting tool with data of a cutting efficiency stored in advance and when the calculated cutting efficiency (e) of the cutting tool is a maximum value among the data of the cutting efficiency (e), using, as the cutting conditions, the feed speed (f) of the cutting tool, the axial direction cutting amount by the cutting tool, the radial direction cutting amount (dr) by the cutting tool, and the cutting speed (v) of the cutting tool, wherein the deflection amount (α) of the cutting tool is calculated by the following expression: α = Fb ave / G, wherein α is the deflection amount (m), Fb ave is an average value of a cutting resistance in a vibrating direction of the cutting tool, and G is a bending rigidity (N/m) in the vibrating direction of the cutting tool. wherein α is the deflection amount (m), Fb ave is an average value of a cutting resistance in a vibrating direction of the cutting tool, and G is a bending rigidity (N/m) in the vibrating direction of the cutting tool.