A waste heat utilization device includes a refrigeration cycle having a compressor, a Rankine cycle using a condenser in common with the refrigeration cycle, and a control unit which controls operation of the refrigeration cycle and the Rankine cycle. The control unit performs a continuation control of the Rankine cycle, in which the Rankine cycle is continuously operated regardless of an operation state of the compressor, when a load of the refrigeration cycle is lower than a predetermined load. In contrast, the control unit performs an intermittent control of the Rankine cycle, in which the Rankine cycle is intermittently operated in accordance with the operation state of the compressor such that the mechanical energy recovered by the expansion unit is larger than a driving energy of the compressor.