We investigate the Gouy phase anomaly of light in the focus of refractive plano-convex microlens arrays by using longitudinal-differential (LD) interferometry and a finite-difference time-domain (FDTD) simulation. We put emphasis on determining the amount of the Gouy phase shift for the line focus of the cylindrical lens and the point focus of the spherical lens. We discuss additional phase structures caused by diffraction, which lead to the discrepancy from the conventional Gouy phase shift.