We investigated effects of magnetic field H on antiferromagnetic (AF) structures in CeRh_{1-x}Co_xIn_5 by performing the elastic neutron scattering measurements. By applying H along the [1,-1,0] direction, the incommensurate AF state with the propaga
tion vector of q_{h1}=(1/2,1/2,0.297) observed at H=0 is replaced by the commensurate AF state with the q_{c2} = (1/2, 1/2, 1/4) modulation above 2 T for x=0.23, while the AF states with the q_{c1}=(1/2,1/2,1/2) and q_{h2}=(1/2,1/2,0.42) modulations seen at H=0 change into a single q_{c1}-AF state above ~1.6 T for x=0.7. These results suggest the different types of AF correlation for Co concentrations of 0.23 and 0.7 in an applied magnetic field H.
