Recently, many researchers have expected millimeter-sized microrobots to work in narrow spaces. However, it is challenging to integrate the actuators, controllers, sensors, and energy sources into millimeter-sized microrobots. Previously, the authors proposed a microrobot that can reproduce ants' tripod walking locomotion and demonstrated that it is activated using a shape memory alloy (SMA) actuator. The SMA provided large deformation and force. However, the SMA actuator consumes a lot of power; therefore, we supplied the power externally. To realize a millimeter-sized microrobots, an actuator that is small and has low power consumption is required. As an alternative means of low-power operation, electrostatic actuators can be used to drive the legs of microrobots. Usually, an electrostatic actuator of a small size is a gap-closing linear-type actuator. Our proposed rotary-type electrostatic motor can output rotational movement using the inchworm mechanism. The inchworm mechanism can rotate the rotor by push motion using multiple linear-type electrostatic actuators. This paper discusses the calculation of theoretical values for driving the legs of the insect-type microrobot using a rotary-type electrostatic motor.