A friction stir spot welding device which welds an object to be welded, by softening the object by friction heat and by stirring the object, the friction stir spot welding device comprising: a tool with a cylindrical shape, the tool being rotatable around an axis thereof, advanceable along a direction of the axis, and retractable along the direction of the axis; characterized by a temperature detector which detects a temperature of the tool; and a controller configured to, in a case where serial spots welding is performed for the object, compare the temperature of the tool which is detected by the temperature detector in a period which passes from end of a welding step for the object until start of a next welding step for the object, to a predetermined reference temperature, to set a plunging force applied by the tool to press the object in the next welding step , wherein the predetermined reference temperature includes a first predetermined reference temperature and a second predetermined reference temperature which is higher than the first predetermined reference temperature, and wherein when the first predetermined reference temperature is designated by T1, the second predetermined reference temperature is designated by T2, the temperature of the tool which is detected by the temperature detector is designated by Tn, the plunging force applied by the tool to press the object at the first predetermined reference temperature is designated by F1, and the plunging force applied by the tool to press the object at the second predetermined reference temperature is designated by F2, the controller is configured to set a plunging force Fn applied by the tool to press the object at the temperature Tn according to a formula Fn = F1 - (Tn - T1)/(T2 - T1) × F2. a tool with a cylindrical shape, the tool being rotatable around an axis thereof, advanceable along a direction of the axis, and retractable along the direction of the axis; characterized by a temperature detector which detects a temperature of the tool; and a controller configured to, in a case where serial spots welding is performed for the object, compare the temperature of the tool which is detected by the temperature detector in a period which passes from end of a welding step for the object until start of a next welding step for the object, to a predetermined reference temperature, to set a plunging force applied by the tool to press the object in the next welding step , wherein the predetermined reference temperature includes a first predetermined reference temperature and a second predetermined reference temperature which is higher than the first predetermined reference temperature, and wherein when the first predetermined reference temperature is designated by T1, the second predetermined reference temperature is designated by T2, the temperature of the tool which is detected by the temperature detector is designated by Tn, the plunging force applied by the tool to press the object at the first predetermined reference temperature is designated by F1, and the plunging force applied by the tool to press the object at the second predetermined reference temperature is designated by F2, the controller is configured to set a plunging force Fn applied by the tool to press the object at the temperature Tn according to a formula Fn = F1 - (Tn - T1)/(T2 - T1) × F2. <EOS>