A roundness measuring apparatus, which has a small space required for installation and of which measurement error due to a temperature change is small, is disclosed. The roundness measuring apparatus includes: a base; a turn-table which is fixed to the base and rotates a work placed on the turn-table; a two-dimensional moving mechanism provided at the base so as to move a holder holding part in parallel to a measurement plane including a rotation axis of the turn-table and a measuring point of the work; a detector holder attached to the holder holding part; and a detector attached to the detector holder so that a probe can be displaced on the measurement plane.