To provide a vehicular opening/closing body locking device which is improved in operability, easy to assemble, and reduced in malfunction.

The vehicular opening/closing body locking device is formed of latch units 8, 9, a control unit 11, a release motor 27, and an outside handle 3. The vehicular opening/closing body locking device is comprised of: a piezoelectric element 21 mounted on the outside handle 3, for detecting pressure applied to the outside handle 3, and converting the pressure into voltage to be output; a step S3 of determining whether the pressure is applied by a person or not, based on at least one of a differential value and a maximum value of the voltage output by the piezoelectric element 21; and steps S7, S8 of driving the release motor 27 to activate the control unit 11, and to cancel locking of the latch units 8, 9 of a slide door 2 if it is determined at the step S3 that the pressure is applied by the person.