Carts and suitcases for carrying luggage are a burden to users, especially when they are carried on stairs. Previous studies on mobile robots with stair-climbing ability focused on legs and/or crawlers. However, many actuators and complex mechanisms are needed for leg locomotion. In propulsion using crawlers, the contact area is limited by size. Other robots use a wheel called Tri-star wheel for climbing stairs. This wheel enables both stair climbing and moving on flat floors. In this study, we propose a power-assisted carrier cart using the Tri-star wheel to reduce the burden of carrying luggage. The drive unit for the carrier cart has three drive wheels on each side, and the center shaft is connected to each drive wheel in each spoke like a planetary gear. Thus, this wheel can move on a plane and climb stairs by rotating the spoke. A load sensor on the handle measures the pulling force of the user to control motors according to the force. Consequently, the user can move the cart within a constant force regardless of load weight and terrain conditions. Herein, we present the design of the proposed carrier cart, simulation to investigate the power-assist control, and experiments conducted to evaluate the power-assist performance using a prototype.