A wire-cut electric discharge machine comprising: a machining vessel having a discharge machining section placed therein for machining a workpiece by discharges by use of a wire electrode; a waste water vessel for collecting and storing a machining fluid from the machining vessel; a freshwater vessel for storing the machining fluid from the waste water vessel after the machining fluid is filtered by a filter; a supply line (L2b) for supplying the machining fluid from the freshwater vessel to upper and lower wire guides (14, 15); an ambient temperature measuring means for measuring the ambient temperature of the wire-cut electric discharge machine; a machining fluid temperature measuring means for measuring the temperature of the machining fluid; a cooling device for controlling the temperature of the machining fluid by cooling the machining fluid in the freshwater vessel and supplying the cooled machining fluid to at least one of the machining vessel, waste water vessel, freshwater vessel, and supply line; and a controller for controlling the cooling operation of the cooling device; characterised in that the controller automatically switches, depending on a relationship between the ambient temperature measured by the ambient temperature measuring means and the machining fluid temperature measured by the machining fluid temperature measuring means, and depending on a relationship between the ambient temperature measured by the ambient temperature measuring means and lowest and highest temperatures in a preset machine ambient temperature range, between a fixed temperature control mode for maintaining the machining fluid temperature at a preset value (Tset), and a differential temperature control mode for causing the machining fluid temperature to follow the ambient temperature so as to limit the difference between the machining fluid temperature and the ambient temperature to a preset range. a machining vessel having a discharge machining section placed therein for machining a workpiece by discharges by use of a wire electrode; a waste water vessel for collecting and storing a machining fluid from the machining vessel; a freshwater vessel for storing the machining fluid from the waste water vessel after the machining fluid is filtered by a filter; a supply line (L2b) for supplying the machining fluid from the freshwater vessel to upper and lower wire guides (14, 15); an ambient temperature measuring means for measuring the ambient temperature of the wire-cut electric discharge machine; a machining fluid temperature measuring means for measuring the temperature of the machining fluid; a cooling device for controlling the temperature of the machining fluid by cooling the machining fluid in the freshwater vessel and supplying the cooled machining fluid to at least one of the machining vessel, waste water vessel, freshwater vessel, and supply line; and a controller for controlling the cooling operation of the cooling device; characterised in that the controller automatically switches, depending on a relationship between the ambient temperature measured by the ambient temperature measuring means and the machining fluid temperature measured by the machining fluid temperature measuring means, and depending on a relationship between the ambient temperature measured by the ambient temperature measuring means and lowest and highest temperatures in a preset machine ambient temperature range, between a fixed temperature control mode for maintaining the machining fluid temperature at a preset value (Tset), and a differential temperature control mode for causing the machining fluid temperature to follow the ambient temperature so as to limit the difference between the machining fluid temperature and the ambient temperature to a preset range.