Optical member, composite optical member, and lighting device

An optical member comprising:

• a first light guide member having an elongated shape and comprising an end surface and a lateral surface extending in a longitudinal direction from the end surface;
• and
• a wavelength conversion layer disposed on the lateral surface of the first light guide member and containing a phosphor, wherein
• the phosphor has a first composition containing an alkali metal containing K, and Si, Al, Mn, and F, wherein
• in the first composition, when a total number of moles of the alkali metal is 2, a total number of moles of Si, Al, and Mn is 0.9 or greater and 1.1 or less, a number of moles of Al is greater than 0 and 0.1 or less, a number of moles of Mn is greater than 0 and 0.2 or less, and a number of moles of F is 5.9 or greater and 6.1 or less, and
• the phosphor has a cubic system crystal structure and a lattice constant of 0.8138 nm or greater.