[PROBLEMS] The present invention provides a novel cleaner having bacteria-proof, fungi-proof and algae-proof effects, which is added to a reaction thermostat using water as medium in scientific apparatus, particularly a thermostatic water bath in an automatic analyzer.[SOUTLIN] A cleaner for a thermostatic water bath, which comprises a quaternary ammonium salt as shown by the general formula [1] described below and/or a preservative agent having amphoteric surface activity, and a surface active agent, ##STR00001## wherein R.sub.1 to R.sub.4 independently represent an alkyl group, provided that at least one among R.sub.1 to R.sub.4 is an alkyl group having 8 to 18 carbon atoms, and at least one among them is a lower alkyl group having 1 to 3 carbon atoms; and X.sup.- represents an anion derived from a carboxylic acid having two or more carbon atoms; and a cleaning method of a thermostatic water bath, wherein the cleaner is added to a thermostatic water bath.