Apparent mass torque coefficients for fluctuations of the flow rate and angular velocity are determined experimentally for two-dimensional centrifugal impellers. Nearly sinusoidal fluctuations of the flow rate and angular velocity are given by using crank mechanisms, and the resulting torque on the impeller is measured. The torque is divided into components in-phase and out-of-phase with the displacements. The in-phase component is attributed to the apparent mass effect and is represented by the apparent mass torque coefficients. Drag torque coefficients are defined and used to represent the out-of-phase components. The apparent mass coefficients are compared with theoretical values obtained under the assumption of a two-dimensional potential flow. The experimental values are 5 to 20 per cent larger than the theoretical ones, and no appreciable effects of the frequency and the amplitude are observed within the range varied in the experiments.