A heat storing device for exchanging heat energy between a heat storing material and a fluid. The heat storing device has a heat storing module having heat storing material spaces filled with a heat storing material and fluid passages for a fluid to flow through adjacent to the heat storing material spaces. The heat storing module includes multiple plates in a stack. Each of the multiple plates has fluid passages in one side thereof. In mutually adjacent pairs of plates, the sides having the fluid passages face each other. When each pair of plates is seen face-on, with respect to the fluid passages of one of the plates, the fluid passages of the other plate intersect substantially at right angles, and the fluid passages of the two plates connect at these positions where they intersect.