A synchronous control device (10) includes a spindle control part (11a) and a driven shaft control part (11b). The driven shaft control part (11b) includes a driven shaft command position variation suppression part (114) which generates as a driven shaft command phase (Ps) a projected driven shaft command phase obtained by adding an advance angle amount (Pa) corresponding to a spindle speed to a spindle phase (Pm) in a case where a sign of the spindle speed is reverse from a previous period, and generates as the driven shaft command phase (Ps) 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.