I understand that, in your example, “just” is an adverb, synonym with ‘only’ and ‘merely’; and adverbs are classified as *content words*, not as *function words*, as you can see in the following references, as well as at the link provided by Peter.

Hope this will of some help,
Manuel
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just
adverb.
1. and nothing more; "I was merely asking"; "it is simply a matter of time"; "just a scratch"; "he was only a child"; "hopes that last but a moment" [syn: merely]
WordNet® 3.0, © 2006 by Princeton University.
<http://dictionary.reference.com/search?q=just&r=66>
<http://dictionary.reference.com/browse/merely>
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(…) For instance, Read (2001) describes the different considerations involved in measuring lexical richness. It is important to understand how a ”word‘ is defined. The first key distinction is between **”function‘ or ”grammatical‘ words such as and, a, to, and this (articles, prepositions, pronouns, conjunctions, auxiliaries etc)** and **”content‘ words such as nouns, verbs, adjectives and adverbs**. Taking the age old example:
<http://www.coe.int/T/DG4/Portfolio/documents/CEF ref supp Se...
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Content items are nouns, full lexical verbs, adjectives, and adverbs. Function items are determiners (articles and quantifiers), pronouns, prepositions, numerals, conjunctions, interjections, negative particles, and auxiliary and modal verbs.
(…)
Determiners (articles and quantifiers), prepositions, pronouns, numerals, conjunctions, interjections, existential “there”, the infinitival marker “to”, negative markers, auxiliary and modal verbs are referred to as “function words” (Sityaev, 2000:294).
<http://www.ohiolink.edu/etd/send-pdf.cgi?osu1117225383>)
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<http://www.ohiolink.edu/etd/send-pdf.cgi?osu1117225383>