A gas sensor controller (100) controls a gas sensor element (10) which has a first pump cell (111), a second pump cell (113), and an oxygen concentration detection cell (112). The controller includes first chamber control means (51) to (53) which pumps oxygen out of or into a first measurement chamber MR1 such that a concentration voltage Vs becomes equal to a target voltage Vr; target voltage changing means S72, S75 which changes the target voltage Vr from a first target voltage Vr1 to a second target voltage Vr2; current detection means (55) which detects the magnitude of a concentration current Ip2 flowing between first electrode (145) and second electrode (147); and deterioration determination means S7 which determines the deterioration state of the gas sensor element from a change which occurs in the concentration current Ip2 due to the change in the target voltage Vr.