What is achieved is an optical wavelength multiplexer/demultiplexer not necessarily requiring the function of adjusting the optical path. A value (ΔLmax−ΔLmin)/L obtained by dividing a difference between a maximum value ΔLmax and a minimum value ΔLmin of ΔL in a range of −40° C. to 80° C. by L is 8×10−6 or less where L represents a length of a crystallized glass (1) at 30° C. and ΔL represents a difference between a length (Lt) of the crystallized glass (1) at each of the temperatures and the length (L) thereof at 30° C.