In order to maintain the pressure of brine inside a separately provided heat exchanging means (4) at at least a saturation pressure for a temperature of vapor 2 output from mechanical vapor compression means (6), even when the circulation of brine is stopped, the invention provides an evaporation chamber (3) for use in a single-stage flash-evaporation fresh water generating apparatus based on mechanical vapor compression, which is disposed at a position higher than the heat exchanging means (4) so as to give a head difference (H) equal to or greater than a difference between the saturation pressure for the temperature of the vapor in the heat exchanging means (4) and the pressure of the evaporation chamber (3), the evaporation chamber including a nozzle (5) which has a shape whose flow-path cross-section gently reduces along the flow direction so that a portion of a static pressure of the bring flowing into the evaporation chamber (3) is efficiently converted to dynamic pressure having a velocity component in the horizontal direction and which is oriented in a direction between 0 and 50 above the horizontal, more preferably between 20° and 50° above; wherein the bottom surface of the evaporation chamber (3) has a horizontal portion (25), which is a portion up to which flash evaporation of decompressed brine (39) passing through the nozzle (5) is substantially completed, and a portion (26) where the bottom surface is inclined at an angle between 20° and 60° above the horizontal, which is a portion where the substantially completely flash-evaporated brine is directed to a brine outlet (24).