A synchronous control device (10, 50), for operating a driven shaft in synchronization with a spindle, comprising: a spindle control part which outputs a spindle command position to a spindle servo control mechanism; and a driven shaft control part which receives a spindle feedback position in the spindle servo control mechanism and outputs a driven shaft command position to a driven shaft servo control mechanism, wherein the driven shaft control part (1 lb) comprises: a phase calculation part which calculates a spindle phase based on the spindle feedback position; a speed calculation part which calculates a spindle speed based on the spindle feedback position; an advance angle amount calculation part which calculates an advance angle amount corresponding to the spindle speed; and a driven shaft command position generation part which generates the driven shaft command position based on a driven shaft command phase, and the driven shaft control part further comprises a driven shaft command position variation suppression part which calculates a projected driven shaft command phase obtained by adding the advance angle amount to the spindle phase, generates the projected driven shaft command phase as the driven shaft command phase in a case where a sign of the spindle speed is reverse from a previous period, and generates as the driven shaft command phase a value in which a sign of a shift from the driven shaft command phase in the previous period is not reverse from the sign of the spindle speed, in a predetermined case where the sign of the spindle speed is not reverse from the previous period, a sign being positive or negative. a spindle control part which outputs a spindle command position to a spindle servo control mechanism; and a driven shaft control part which receives a spindle feedback position in the spindle servo control mechanism and outputs a driven shaft command position to a driven shaft servo control mechanism, wherein the driven shaft control part (1 lb) comprises: a phase calculation part which calculates a spindle phase based on the spindle feedback position; a speed calculation part which calculates a spindle speed based on the spindle feedback position; an advance angle amount calculation part which calculates an advance angle amount corresponding to the spindle speed; and a driven shaft command position generation part which generates the driven shaft command position based on a driven shaft command phase, and the driven shaft control part further comprises a driven shaft command position variation suppression part which calculates a projected driven shaft command phase obtained by adding the advance angle amount to the spindle phase, generates the projected driven shaft command phase as the driven shaft command phase in a case where a sign of the spindle speed is reverse from a previous period, and generates as the driven shaft command phase a value in which a sign of a shift from the driven shaft command phase in the previous period is not reverse from the sign of the spindle speed, in a predetermined case where the sign of the spindle speed is not reverse from the previous period, a sign being positive or negative. calculates a projected driven shaft command phase obtained by adding the advance angle amount to the spindle phase, generates the projected driven shaft command phase as the driven shaft command phase in a case where a sign of the spindle speed is reverse from a previous period, and generates as the driven shaft command phase a value in which a sign of a shift from the driven shaft command phase in the previous period is not reverse from the sign of the spindle speed, in a predetermined case where the sign of the spindle speed is not reverse from the previous period, a sign being positive or negative.