An in-liquid-substance detection sensor that achieves size reduction and detection accuracy improvement includes a piezoelectric substrate, at least two SAW devices provided on one major surface of the piezoelectric substrate and each having at least one IDT electrode defining a sensing portion, outer electrodes provided on the other major surface of the piezoelectric substrate and electrically connected to the SAW devices through vias extending through the piezoelectric substrate, a channel-defining member provided on the one major surface of the piezoelectric substrate so as to surround the SAW devices and a region connecting the SAW devices to each other, thereby defining sidewalls of a channel, and a protective member bonded to the one major surface of the piezoelectric substrate with the channel-defining member interposed therebetween, thereby sealing the channel, the protective member having at least two through holes communicating with the channel.