To provide a device for controlling the speed of a DC motor which can set an arbitrary constant or variable target rotational speed and can perform speed control such that an actual rotational speed well matches the target rotational speed even in a structure in which a supply voltage fluctuates and a load varies.

The device for controlling the speed of a DC motor of the present invention includes: a rotational speed detection circuit for detecting a rotational speed of the DC motor and outputting a detection pulse corresponding to the rotational speed; a target rotational speed setting circuit for outputting a target pulse corresponding to a target rotational speed; a command operation circuit for comparing the detection pulse with the target pulse and outputting a command signal Vref corresponding to a rotational speed difference that is the rotational speed minus the target rotational speed; and a step-up/down power supply section 6 for outputting as a current DC driving voltage VM a DC voltage stepped up/down on the basis of the command signal Vref and a past DC driving voltage (an index voltage Vm).