Traffic restriction is required for replacement of existing bridge deck slabs. To reduce the inconvenience, there has been a strong need for minimizing the construction time. The construction time tends to be longer when precast prestressed concrete slabs are used because it is necessary to move each slab placed in a temporary position to the right position by using lever hoists or the like. This study proposes an innovative mechanical joint that can shorten the time of slab installation. The proposed joint consists of a steel cotter and a pair of steel holders with longitudinal anchors. The holders are embedded in the slabs, and each slab placed in a temporary position can be repositioned to the right place and joined to its adjacent slab by simply bolting the cotter to the holders. This simple step can reduce the installation time per slab in half. The gap of 30 mm left between two slabs is filled with non-shrink mortar. To study the flexural capacity of the joint, static flexural tests were carried out before and after improvement of the design of the cotter. It was shown that the flexural capacity of the junction using the improved cotter was higher than the yield load of the reinforcement of the precast slabs, which was thought to ensure compressive failure of the concrete at the junction. It was also found that the longitudinal stress in the cotter at the center of the junction could be estimated by applying the beam theory. The tensile force of the steel cotter due to bending was found to be transmitted to the longitudinal reinforcement through the anchors of the steel holders, exhibiting an adequate flexural capacity under the test conditions. It was also revealed that the beam theory could be used to estimate the stress in the longitudinal reinforcement or the anchors.