To suppress output changes of an angular acceleration according to a pitch error of an inductor or a slit plate.

In this angular acceleration measuring device, a pair of first velocity pulse generators 4-1, 4-2 are provided on a first center lines 15 of a rotation shaft close to an inductor 3 attached to the rotation shaft, and a pair of second velocity pulse generators 4-3, 4-4 are provided on a second center line 16 inclined at 180/m degrees wherein (m) is the number of output changes of an angular acceleration. Outputs of the respective velocity pulse generators from 4-1 to 4-4 are shapered by pulse shaping circuits 5-1, 5-2, then counted by velocity pulse counters 7-1, 7-2, and period measuring clocks are counted by period measuring counter 8-1, 8-2. The respective counted numbers are stored in memories 9, angular velocity calculation devices 10 calculate average angular velocities, angular acceleration calculation devices 12 respectively differentiate the average angular velocities to calculate angular accelerations, and an average value calculation device 18 averages outputs of the two angular acceleration devices 12.