The efficiency and mechanism of a microbubble washing method for a flat surface placed parallel to the direction of an ultrasound wave (i.e., a side surface) were investigated. After 20 intermittent ultrasonic washing steps with microbubbles, 89.7 +/- 4.0% of the coated press oil on the side surface was removed. The efficiency was slightly lower than that for a flat surface placed perpendicular to the direction of the ultrasound wave (i.e., a bottom surface, 92.9 +/- 4.4%). However, the washing efficiency for the side surface was sufficient to meet guidelines for press oil contamination on the surfaces of products manufactured by machine presses. Observations of microbubble movements with a high-speed camera were used to propose a mechanism for washing of the side surface, and this mechanism differed from that for the bottom surface.