To provide a measuring apparatus, for the insulation of a hot line, by which the insulation can be measured without depending on commercial frequency, which reduces the influence of a change in an output signal when a noise or the like enters and by which a phase shift is corrected correctly so that a phase discrimination function acts effectively.

On the basis of a switching signal 105, a switching filter 6 passes a signal only for a period 1/fo. A phase correction circuit 5 corrects the phase shift on the basis of a reference reactive signal 102. A section averaging circuit 8 and an output hold circuit 9 cut off an input-side signal route and an output-side signal route in a period in which the output signal 109 is easily influenced, and they send a held value to the output side.