In recent years, isothermal nucleic acid amplification techniques have been developed as alternatives to polymerase chain reaction (PCR) requiring thermal cycles. The integration of isothermal amplification into electrical or electrochemical devices realizes high throughput nucleic acid-based assays and confers high sensitivity. In this research, we tried to develop electrical/electrochemical biosensors detecting nucleic acids aiming at application to future liquid biopsy. For detection of small DNAs or microRNAs (miRNAs), isothermal amplification methods were employed, and the signal changes based on the extension reaction by polymerases were detected using electrical/electrochemical devices, namely, a chronocoulometric sensor and a micro pH sensor. These small sensors based on electrochemical technique might be a promising tool in on-site detection of nucleic acids related to cancer diagnosis.