A circuit in which a storage function and an arithmetic function are combined is proposed by using a transistor with low off-state current for forming a storage element. When the transistor with low off-state current is used, electric charge can be held, for example, in a node or the like between a source or a drain of the transistor with low off-state current and a gate of another transistor. Thus, the node or the like between one of the source or the drain of the transistor with low off-state current and the gate of the another transistor can be used as a storage element. In addition, leakage current accompanied by the operation of an adder can be reduced considerably. Accordingly, a signal processing circuit consuming less power can be formed.