A resonant DC-DC converter including a control device, the resonant DC-DC converter including: a direct current power supply (E d); a full-bridge circuit of which the input side is connected to both ends of the direct current power supply (E d) and to an output side of which a primary coil (N P) of a transformer (T r) is connected via a series resonant circuit (L r, C r), and which is configured of semiconductor switching elements (Q 1, Q 2, Q 3, Q 4); a rectifier circuit connected to a secondary coil (N s) of the transformer (T r); and a smoothing capacitor (C o) connected to an output side of the rectifier circuit, wherein, by the semiconductor switching elements (Q 1, Q 2, Q 3, Q 4) being turned on and off to cause a resonance current to flow through the series resonant circuit (L r, C r), a direct current output voltage V out is output via the transformer (T r), the rectifier circuit, and the smoothing capacitor (C o), the control device comprising: a means for detecting a quantity of electricity in accordance with a status of a load of the resonant DC-DC converter and determining a control amount for controlling the turning on and off of the semiconductor switching elements (Q 1, Q 2, Q 3, Q 4); a pulse distribution means for generating drive pulses G 1, G 2, G 3, G 4 of the semiconductor switching elements (Q 1, Q 2, Q 3, Q 4); and the control device being characterized by further comprising a frequency modulation control means for executing a frequency modulation control of the semiconductor switching elements (Q 1, Q 2, Q 3, Q 4) at a frequency F s lower than a resonance frequency F r of the series resonant circuit based on the control amount; a fixed frequency control means for executing a fixed frequency control of the semiconductor switching elements (Q 1, Q 2, Q 3, Q 4) at a frequency F s in a vicinity of the resonance frequency F r when the control amount is smaller than an upper limit value; and the pulse distribution means generating the drive pulses G 1, G 2, G 3, G 4 of the semiconductor switching elements based on outputs of the frequency modulation control means and the fixed frequency control means, wherein, when the direct current output voltage V out of the resonant DC-DC converter reaches a value that exceeds a maximum value that can be output in a fixed frequency control region, the control amount causes switching from a control operation by the fixed frequency control means to a control operation by the frequency modulation control means. a direct current power supply (E d); a full-bridge circuit of which the input side is connected to both ends of the direct current power supply (E d) and to an output side of which a primary coil (N P) of a transformer (T r) is connected via a series resonant circuit (L r, C r), and which is configured of semiconductor switching elements (Q 1, Q 2, Q 3, Q 4); a rectifier circuit connected to a secondary coil (N s) of the transformer (T r); and a smoothing capacitor (C o) connected to an output side of the rectifier circuit, wherein, by the semiconductor switching elements (Q 1, Q 2, Q 3, Q 4) being turned on and off to cause a resonance current to flow through the series resonant circuit (L r, C r), a direct current output voltage V out is output via the transformer (T r), the rectifier circuit, and the smoothing capacitor (C o), a means for detecting a quantity of electricity in accordance with a status of a load of the resonant DC-DC converter and determining a control amount for controlling the turning on and off of the semiconductor switching elements (Q 1, Q 2, Q 3, Q 4); a pulse distribution means for generating drive pulses G 1, G 2, G 3, G 4 of the semiconductor switching elements (Q 1, Q 2, Q 3, Q 4); and a frequency modulation control means for executing a frequency modulation control of the semiconductor switching elements (Q 1, Q 2, Q 3, Q 4) at a frequency F s lower than a resonance frequency F r of the series resonant circuit based on the control amount; a fixed frequency control means for executing a fixed frequency control of the semiconductor switching elements (Q 1, Q 2, Q 3, Q 4) at a frequency F s in a vicinity of the resonance frequency F r when the control amount is smaller than an upper limit value; and the pulse distribution means generating the drive pulses G 1, G 2, G 3, G 4 of the semiconductor switching elements based on outputs of the frequency modulation control means and the fixed frequency control means, wherein, when the direct current output voltage V out of the resonant DC-DC converter reaches a value that exceeds a maximum value that can be output in a fixed frequency control region, the control amount causes switching from a control operation by the fixed frequency control means to a control operation by the frequency modulation control means.