Photophysical properties and photocatalytic activities for water splitting over KM2Nb5O15 (M=Sr and Ba) and K2LnNb5O15 (Ln=La, Pr, Nd, and Sm) with tungsten bronze-type structure were investigated. Single phases of KM2Nb5O15 and K2LnNb5O15 were successfully prepared by solid-state reaction (SSR) method and polymerizable complex (PC) method. The band gaps of these niobates were estimated to be 3.1–3.5 eV. These metal oxides loaded with an activated NiOx cocatalyst showed photocatalytic activities for water splitting into H2 and O2 under UV irradiation. When K2LaNb5O15 was doped by Rh ions, a new visible-light absorption band was observed around 400–500 nm in addition to the band gap absorption band of the K2LaNb5O15 host. K2LaNb5O15:Rh showed photocatalytic activities for H2 or O2 evolution from an aqueous solution containing a sacrificial reagent under visible-light irradiation.