PURPOSE: To shorten a focusing time by using one condenser lens for both irradiating laser light and condensing reflected light, and inputting reflected light of a dichroic mirror and transmitting light to a temperature measuring system and a flow velocity measuring system, respectively.

CONSTITUTION: Laser light 104 of a wavelength 5,145 oscillated from a laser light source 103 is divided into two by a beam splitter 105, and crossed by a condenser lens 106. An interference fringe by two laser lights 104 is formed on the crossing part of a measuring position 107. Incident light 108 condensed by the lens 106 is made spectral by a dichroic mirror 109, and transmitted scattered light 108a transmits through an interference filter 111 through a mirror 110 and photoelectrically converted by a photomultiplier 112. Said light becomes a flow velocity signal 115 proportional to a flow velocity of a high temperature fine particle 102 by a flow velocity signal processing device 114, and a flow velocity is derived by a computer 116. As soon as a flow velocity is measured by an A channel, radiant light 108b reflected by the dichroic mirror 109 is brought to signal processing in a B channel, and the intensity of the radiant light is measured.