  We have identified a weak thermal line U42.767, which has been detected only
in the directions toward Sgr A and Sgr B2, as the HOCO+ 2_{02}--1_{01}
transition. Because of the proximity of this line to the SiO maser line at
42.821 GHz (J=1-0 v=2),it was observable simultaneously in the ~43 GHz SiO
maser searches at Nobeyama. From the past data of SiO maser surveys of infrared
objects in the Galactic center, we created a map of emission distribution of
HOCO+ in the Sgr A molecular cloud as well as maps of the 29SiO J=1-0 v=0
thermal emission and H53alpha emission. The emission distribution of HOCO+ was
quite similar to the distribution of 29SiO emission. It suggests that the
enhancement of the HOCO+ abundance in the galactic center is induced by shock
activities which release the CO2 molecules frozen on grains into gases.
