Mixed Reality (MR) technologies have recently been explored in many areas of Human-Machine Interface (HMI) such as medicine, manufacturing, entertainment and education. However, MR sickness, a kind of motion sickness, is caused by sensory conflicts between the real world and the virtual world. The purpose of this paper is to determine a method to reduce MR sickness associated with MR technology with a Head-Up Display (HUD). This MR environment is modeled after advanced driver assistance systems in near-future vehicles. In this paper, we evaluate the influence of the delay time between real images and virtual images derived from image processing in a MR system on equilibrium function and activity of the autonomous nervous system (respiration). The seated subjects in the MR simulator were shaken in the pitch direction at 0.1, 0.2, 0.4, 1.0 and 2.0 Hz and presented with 10-min MR simulations at three delay times: about 66 ms, 133 ms and 200 ms. We discovered that the biological effect of the MR environment increases or has peak shift as the delay time becomes larger. This tendency is particularly strong in the range of low frequencies, under 0.4 Hz or 1.0 Hz.