  Protostellar outflows are considered a signpost of star formation. These
outflows can cause shocks in the molecular gas and are typically traced by the
line wings of certain molecules. HNCO (4--3) has been regarded as a shock
tracer because of the high abundance in shocked regions. Here we present the
first imaging results of HNCO (4--3) line wings toward nine sources in a sample
of twenty three massive star-forming regions using the IRAM 30\,m telescope. We
adopt the velocity range of the full width of HC$_{3}$N (10--9) and
H$^{13}$CO$^+$ (1--0) emissions as the central emission values, beyond which
the emission from HNCO (4--3) is considered to be from line wings. The spatial
distributions of the red- and/or blue-lobes of HNCO (4--3) emission nicely
associate with those lobes of HCO$^{+}$ (1--0) in most of the sources. High
intensity ratios of HNCO (4--3) to HCO$^+$ (1--0) are obtained in the line
wings. The derived column density ratios of HNCO to HCO$^+$ are consistent with
those previously observed towards massive star-forming regions. These results
provide direct evidence that HNCO could trace outflow in massive star-forming
regions. This work also implies that the formation of some HNCO molecules is
related to shock, either on the grain surface or within the shocked gas.
