To provide a variable diaphragm device for an endoscope having a simple constitution, capable of easily achieving miniaturization thereof so as to be used for an endoscope objective optical system, and also, capable of accurately obtaining an intermediate open degree (diaphragm value).

The variable diaphragm device includes: a freely expandable donut-shaped balloon 7 formed of an elastic material into a donut shape, whose inside diameter functions as an aperture diaphragm; and a balloon internal-pressure control means 10 for controlling the internal pressure of the donut-shaped balloon 7. As the internal pressure of the donut-shaped balloon 7 is increased by the balloon internal-pressure control means 10, the inside diameter of the donut-shaped balloon 7 is decreased, as the internal pressure of the donut-shaped balloon 7 is decreased, the inside diameter of the donut-shaped balloon 7 is increased.