Disclosed is an open-junction ionic transistor which includes: a substrate; a p type gel which is formed as a polyelectrolyte gel on the substrate; an n type gel which is formed as the polyelectrolyte gel on the substrate and having one side contacting one side of the p type gel; a first reservoir contacting the other side of the p type gel; a second reservoir contacting the other side of the n type gel; and an encapsulation layer covering the p type gel, the n type gel, the first reservoir, and the second reservoir, in which on the encapsulation layer, an injection unit for injecting an ion input is formed at a location corresponding to an interface contacting the p type gel and the n type gel and when reverse bias voltage is applied between the p type gel and the n type gel, the ion input injected through the injection unit is amplified and ionic current peak is generated.