A control device for an electric compressor for use in a refrigeration cycle includes an inverter circuit and controller for outputting a driving signal to a DC motor. The controller is configured to set a specified constant rotating speed as a target rotating speed (Rt) of the DC motor (103) (SI), obtain an actual measurement rotating speed (Rm) which is a measurement value of a rotating speed of the DC motor (103) (S2), adjust a duty ratio of driving electric power of the DC motor (103) so that the actual measurement rotating speed (Rm) matches the target rotating speed (Rt) (S3), and newly set the target rotating speed based on a change in the duty ratio (S4).