A deposit measurement apparatus intended to be applied to a heat exchanger provided with a heat-conducting pipe and a pipe-supporting plate in which a through-hole for inserting the heat-conducting pipe is formed, the through-hole having N protrusions in a circumferential direction of the heat-conducting pipe, in order to estimate a blockage ratio of a gap that is formed between protrusions and that exists between a wall surface of the through-hole and an outer surface of the heat-conducting pipe, or a thickness of a deposit deposited at said gap,the deposit measurement apparatus comprising: signal acquisition means comprising a single or a plurality of sensors for acquiring a flaw detection signal by scanning inside the heat-conducting pipe with said a single or a plurality of sensors; and processing means for estimating the blockage ratio at the gap or the thickness of the deposit deposited at the gap by using the flaw detection signal acquired only at said gap, characterized in that the signal acquisition means is arranged for acquiring flaw detection signals when M measurement points disposed inside the heat-conducting pipe at equal intervals in the circumferential direction thereof are scanned in the axial direction of the heat-conducting pipe by said single or plurality of sensors provided inside the heat-conducting pipe; and in that the processing means includes: group-representative-signal calculating means for forming groups so that the flaw detection signals from the measuring points located every M/N measuring points are placed in the same groups and for determining a representative signal for each group; evaluation-group selection means for specifying protrusion groups that can be considered to be positioned at the pipe-supporting-plate protrusions in the through-hole on the basis of the representative signals and for selecting signals from evaluation groups, which are groups other than the protrusion groups, as evaluation sensor signals; and estimating means for estimating blockage ratios or thicknesses of deposits by using the evaluation sensor signals. signal acquisition means comprising a single or a plurality of sensors for acquiring a flaw detection signal by scanning inside the heat-conducting pipe with said a single or a plurality of sensors; and processing means for estimating the blockage ratio at the gap or the thickness of the deposit deposited at the gap by using the flaw detection signal acquired only at said gap, in that the processing means includes: group-representative-signal calculating means for forming groups so that the flaw detection signals from the measuring points located every M/N measuring points are placed in the same groups and for determining a representative signal for each group; evaluation-group selection means for specifying protrusion groups that can be considered to be positioned at the pipe-supporting-plate protrusions in the through-hole on the basis of the representative signals and for selecting signals from evaluation groups, which are groups other than the protrusion groups, as evaluation sensor signals; and estimating means for estimating blockage ratios or thicknesses of deposits by using the evaluation sensor signals.