According to an embodiment, an adder includes first and second wave computing units and a threshold wave computing unit. Each of the first and second wave computing units includes a pair of first input sections, a first wave transmission medium having a continuous film including a magnetic body connected to the first input sections, and a first wave detector outputting a result of computation by spin waves induced in the first wave transmission medium by the signals corresponding to the two bit values. The threshold wave computing unit includes a plurality of third input sections, a third wave transmission medium having a continuous film including a magnetic body connected to the third input sections, and a third wave detector a result of computation by spin waves induced in the third wave transmission medium.