To provide a brake liquid composition based on a polyethermonool and having a high boiling point, a low freezing temperature, and a reduced tendency to swell rubber.

This brake liquid composition comprises 90-99 mass % (A) polyether monool of a number-average molecular weight of 160-300 represented by the formula (1): R1-O-[(EO)m/(PO)n]-H and 1-10 mass % (B) glycol represented by formula (2): HO-(EO)k-H [in formulae (1) and (2), R1 is a 1-4C alkyl, or the like; E is ethylene group: P is 1,2-proylene group; (m) and (n) are each an integer of 0 or greater provided that (m) is 2-5 on the average, and (n) is 0.65-1.25 on the average; and (k) is an integer of 1 or greater provided that (k) is 1-6 on the average] and has a boiling point of at least 240°C, a freezing temperature of -50°C or below, and a swelling volume of at most 13% vol., as measured on a rubber (SBR) according to JIS K 2233.