An electrical power supply system comprising: a DC electrical power source having a source voltage VB; a boost converter which includes a switching device (Q1; Q2) and which is configured to boost the source voltage VB by a predetermined boost control and to output the boosted voltage to a load apparatus, the boost control including a change of a switching state of the switching device (Q1; Q2) based on a boost instruction voltage; and a voltage detecting device which is arranged to detect an output voltage VH of the boost converter, and a control apparatus comprising: a boost controlling device configured to perform the boost control; and an intermittent controlling device configured to perform a process of intermittent boost control based on the detected output voltage VH to keep the output voltage VH within a range including the boost instruction voltage which is used when the last boost control is performed, characterized in that the intermittent controlling device is configured to start the process of intermittent boost control when it is determined that a range of a variation of the output voltage VH is within a predetermined value for a predetermined time, or the control apparatus further comprises an output variation specifying device which specifies a change rate of an output of the DC electrical power source or the load apparatus, the intermittent controlling device being configured to start the process of intermittent boost control when the specified change rate of the output is within a predetermined value. a DC electrical power source having a source voltage VB; a boost converter which includes a switching device (Q1; Q2) and which is configured to boost the source voltage VB by a predetermined boost control and to output the boosted voltage to a load apparatus, the boost control including a change of a switching state of the switching device (Q1; Q2) based on a boost instruction voltage; and a voltage detecting device which is arranged to detect an output voltage VH of the boost converter, and a control apparatus comprising: a boost controlling device configured to perform the boost control; and an intermittent controlling device configured to perform a process of intermittent boost control based on the detected output voltage VH to keep the output voltage VH within a range including the boost instruction voltage which is used when the last boost control is performed, characterized in that a boost controlling device configured to perform the boost control; and an intermittent controlling device configured to perform a process of intermittent boost control based on the detected output voltage VH to keep the output voltage VH within a range including the boost instruction voltage which is used when the last boost control is performed, characterized in that the intermittent controlling device is configured to start the process of intermittent boost control when it is determined that a range of a variation of the output voltage VH is within a predetermined value for a predetermined time, or the control apparatus further comprises an output variation specifying device which specifies a change rate of an output of the DC electrical power source or the load apparatus, the intermittent controlling device being configured to start the process of intermittent boost control when the specified change rate of the output is within a predetermined value.