An observation apparatus comprises a thermoregulation mechanism including a liquid receiving frame (28) that has a heater (43) for heating the outer frame of an immersion objective lens (13) and adapted to heat and maintain the temperature of the immersion liquid (30) contained in the frame (28) by means of the heater (43) and via the outer frame of the immersion objective lens (13) so as to maintain the temperature of the specimen (34) in the container at a constant level by utilizing heat conduction. The thermoregulation mechanism of the observation apparatus is so adapted that, when the temperature of the immersion liquid (30) decreases during a measurement/observation operation due to discharge of heat, the liquid (30) is heated again by the heater (43) to prevent the temperature of the specimen (34) from changing and maintain the temperature at a constant level.