  The first-known tidal disruption event (TDE) with strong evidence for a
relativistic jet -- based on extensive multi-wavelength campaigns -- is Swift
J1644+5734. In order to directly measure the apparent speed of the radio jet,
we performed very long baseline interferometry (VLBI) observations with the
European VLBI network (EVN) at 5 GHz. Our observing strategy was to identify a
very nearby and compact radio source with the real-time e-EVN, and then utilise
this source as a stationary astrometry reference point in the later five deep
EVN observations. With respect to the in-beam source FIRST J1644+5736, we have
achieved a statistical astrometric precision about 12 micro-arcsecond (68 %
confidence level) per epoch. This is one of the best phase-referencing
measurements available to date. No proper motion has been detected in the Swift
J1644+5734 radio ejecta. We conclude that the apparent average ejection speed
between 2012.2 and 2015.2 was less than 0.3c with a confidence level of 99 %.
This tight limit is direct observational evidence for either a very small
viewing angle or a strong jet deceleration due to interactions with a dense
circum-nuclear medium, in agreement with some recent theoretical studies.
