To provide an elevating device which can move a sump strainer easily from a sump opening in the bottom of a cold water tank to a platform above it and from the platform to the sump opening while preventing drop of foreign matters.

A first chain 23 fitted to the end side of a left strainer element 21a in a sump strainer 21 made up of a plurality of strainer elements 21a, 21b and 21c connected foldably by hinges 22, a second chain 24 fitted to the end side of the left strainer element 21a and also inserted through ring-shaped metal fittings 26 provided on the end side of the right strainer element 21c and a pair of strainer elevation guide members 28 disposed at a platform provided above the sump strainer 21, virtually at the same interval as the dimension of the width of the sump strainer 21, are provided. The opposite end parts of a second end part of the strainer element on the other end side are engaged slidably with rails formed on the strainer elevation guide members 28.