A deep body thermometer that includes a body temperature measurement unit having a thermal resistor layer formed of a thermal resistor having a predetermined thermal resistance value, four temperature sensors arranged in a thickness direction of the thermal resistor layer, and a wiring substrate on which a processing circuit for processing an output signal of each of the four temperature sensors is mounted. Moreover, the thermometer includes an upper exterior body made of a foamed material of a closed cell or a semi-closed cell having a waterproof property to accommodate the body temperature measurement unit, and a lower exterior body formed of a non-foamed resin film having a waterproof property, in which peripheral edge portions of the upper exterior body and the lower exterior body are fixed in a close contact manner.